diff --git a/.ci-builds/.azure-pipelines.yml b/.ci-builds/.azure-pipelines.yml
index 675323fae62..a35cc3586b7 100644
--- a/.ci-builds/.azure-pipelines.yml
+++ b/.ci-builds/.azure-pipelines.yml
@@ -37,6 +37,19 @@ jobs:
         CC: clang
         CXX: clang++
 #-------------------------------------------------------------------------
+- job: ICU4C_Clang_Ubuntu_1604_WarningsAsErrors
+  displayName: 'C: Linux Clang WarningsAsErrors (Ubuntu 16.04)'
+  timeoutInMinutes: 30
+  pool:
+    vmImage: 'Ubuntu 16.04'
+  steps:
+    - script: |
+        export CPPFLAGS="-Werror -Wall -Wextra -Wextra-semi" && cd icu4c/source && ./runConfigureICU Linux && make -j2 tests
+      displayName: 'Build only (WarningsAsErrors)'
+      env:
+        CC: clang
+        CXX: clang++
+#-------------------------------------------------------------------------
 - job: ICU4C_Clang_Ubuntu_DataFilter_1604
   displayName: 'C: Linux Clang DataFilter (Ubuntu 16.04)'
   timeoutInMinutes: 30
diff --git a/icu4c/source/common/mutex.h b/icu4c/source/common/mutex.h
index d8fdf285b46..44b1f90ba04 100644
--- a/icu4c/source/common/mutex.h
+++ b/icu4c/source/common/mutex.h
@@ -57,10 +57,10 @@ class U_COMMON_API Mutex : public UMemory {
 public:
     Mutex(UMutex *mutex = nullptr) : fMutex(mutex) {
         umtx_lock(fMutex);
-    };
+    }
     ~Mutex() {
         umtx_unlock(fMutex);
-    };
+    }
 
     Mutex(const Mutex &other) = delete; // forbid assigning of this class
     Mutex &operator=(const Mutex &other) = delete; // forbid copying of this class
diff --git a/icu4c/source/common/umutex.h b/icu4c/source/common/umutex.h
index d6164f955ce..13e38b49aae 100755
--- a/icu4c/source/common/umutex.h
+++ b/icu4c/source/common/umutex.h
@@ -225,7 +225,7 @@ public:
         if (m == nullptr) { m = getMutex(); }
         m->lock();
     }
-    void unlock() { fMutex.load(std::memory_order_relaxed)->unlock(); };
+    void unlock() { fMutex.load(std::memory_order_relaxed)->unlock(); }
 
     static void cleanup();
 
diff --git a/icu4c/source/test/cintltst/capitst.c b/icu4c/source/test/cintltst/capitst.c
index bf751b1aa2d..18d925d096f 100644
--- a/icu4c/source/test/cintltst/capitst.c
+++ b/icu4c/source/test/cintltst/capitst.c
@@ -42,6 +42,7 @@ static void TestBengaliSortKey(void);
 
 
 static char* U_EXPORT2 ucol_sortKeyToString(const UCollator *coll, const uint8_t *sortkey, char *buffer, uint32_t len) {
+    (void)coll; // suppress compiler warnings about unused variable
     uint32_t position = 0;
     uint8_t b;
 
@@ -530,8 +531,8 @@ void TestRuleBasedColl()
     }
     while (TRUE) {
         /* testing with en since thai has its own tailoring */
-        uint32_t ce = ucol_next(iter1, &status);
-        uint32_t ce2 = ucol_next(iter2, &status);
+        int32_t ce = ucol_next(iter1, &status);
+        int32_t ce2 = ucol_next(iter2, &status);
         if(U_FAILURE(status)) {
             log_err("ERROR: CollationElement iterator creation failed.: %s\n", myErrorName(status));
             return;
@@ -1544,64 +1545,64 @@ void TestBounds() {
     };
 
     struct teststruct tests[] = {
-        {"\\u010CAKI MIHALJ" } ,
-        {"\\u010CAKI MIHALJ" } ,
-        {"\\u010CAKI PIRO\\u0160KA" },
-        {"\\u010CABAI ANDRIJA" } ,
-        {"\\u010CABAI LAJO\\u0160" } ,
-        {"\\u010CABAI MARIJA" } ,
-        {"\\u010CABAI STEVAN" } ,
-        {"\\u010CABAI STEVAN" } ,
-        {"\\u010CABARKAPA BRANKO" } ,
-        {"\\u010CABARKAPA MILENKO" } ,
-        {"\\u010CABARKAPA MIROSLAV" } ,
-        {"\\u010CABARKAPA SIMO" } ,
-        {"\\u010CABARKAPA STANKO" } ,
-        {"\\u010CABARKAPA TAMARA" } ,
-        {"\\u010CABARKAPA TOMA\\u0160" } ,
-        {"\\u010CABDARI\\u0106 NIKOLA" } ,
-        {"\\u010CABDARI\\u0106 ZORICA" } ,
-        {"\\u010CABI NANDOR" } ,
-        {"\\u010CABOVI\\u0106 MILAN" } ,
-        {"\\u010CABRADI AGNEZIJA" } ,
-        {"\\u010CABRADI IVAN" } ,
-        {"\\u010CABRADI JELENA" } ,
-        {"\\u010CABRADI LJUBICA" } ,
-        {"\\u010CABRADI STEVAN" } ,
-        {"\\u010CABRDA MARTIN" } ,
-        {"\\u010CABRILO BOGDAN" } ,
-        {"\\u010CABRILO BRANISLAV" } ,
-        {"\\u010CABRILO LAZAR" } ,
-        {"\\u010CABRILO LJUBICA" } ,
-        {"\\u010CABRILO SPASOJA" } ,
-        {"\\u010CADE\\u0160 ZDENKA" } ,
-        {"\\u010CADESKI BLAGOJE" } ,
-        {"\\u010CADOVSKI VLADIMIR" } ,
-        {"\\u010CAGLJEVI\\u0106 TOMA" } ,
-        {"\\u010CAGOROVI\\u0106 VLADIMIR" } ,
-        {"\\u010CAJA VANKA" } ,
-        {"\\u010CAJI\\u0106 BOGOLJUB" } ,
-        {"\\u010CAJI\\u0106 BORISLAV" } ,
-        {"\\u010CAJI\\u0106 RADOSLAV" } ,
-        {"\\u010CAK\\u0160IRAN MILADIN" } ,
-        {"\\u010CAKAN EUGEN" } ,
-        {"\\u010CAKAN EVGENIJE" } ,
-        {"\\u010CAKAN IVAN" } ,
-        {"\\u010CAKAN JULIJAN" } ,
-        {"\\u010CAKAN MIHAJLO" } ,
-        {"\\u010CAKAN STEVAN" } ,
-        {"\\u010CAKAN VLADIMIR" } ,
-        {"\\u010CAKAN VLADIMIR" } ,
-        {"\\u010CAKAN VLADIMIR" } ,
-        {"\\u010CAKARA ANA" } ,
-        {"\\u010CAKAREVI\\u0106 MOMIR" } ,
-        {"\\u010CAKAREVI\\u0106 NEDELJKO" } ,
-        {"\\u010CAKI \\u0160ANDOR" } ,
-        {"\\u010CAKI AMALIJA" } ,
-        {"\\u010CAKI ANDRA\\u0160" } ,
-        {"\\u010CAKI LADISLAV" } ,
-        {"\\u010CAKI LAJO\\u0160" } ,
-        {"\\u010CAKI LASLO" } ,
+        {"\\u010CAKI MIHALJ", {0}},
+        {"\\u010CAKI MIHALJ", {0}},
+        {"\\u010CAKI PIRO\\u0160KA", {0}},
+        {"\\u010CABAI ANDRIJA", {0}},
+        {"\\u010CABAI LAJO\\u0160", {0}},
+        {"\\u010CABAI MARIJA", {0}},
+        {"\\u010CABAI STEVAN", {0}},
+        {"\\u010CABAI STEVAN", {0}},
+        {"\\u010CABARKAPA BRANKO", {0}},
+        {"\\u010CABARKAPA MILENKO", {0}},
+        {"\\u010CABARKAPA MIROSLAV", {0}},
+        {"\\u010CABARKAPA SIMO", {0}},
+        {"\\u010CABARKAPA STANKO", {0}},
+        {"\\u010CABARKAPA TAMARA", {0}},
+        {"\\u010CABARKAPA TOMA\\u0160", {0}},
+        {"\\u010CABDARI\\u0106 NIKOLA", {0}},
+        {"\\u010CABDARI\\u0106 ZORICA", {0}},
+        {"\\u010CABI NANDOR", {0}},
+        {"\\u010CABOVI\\u0106 MILAN", {0}},
+        {"\\u010CABRADI AGNEZIJA", {0}},
+        {"\\u010CABRADI IVAN", {0}},
+        {"\\u010CABRADI JELENA", {0}},
+        {"\\u010CABRADI LJUBICA", {0}},
+        {"\\u010CABRADI STEVAN", {0}},
+        {"\\u010CABRDA MARTIN", {0}},
+        {"\\u010CABRILO BOGDAN", {0}},
+        {"\\u010CABRILO BRANISLAV", {0}},
+        {"\\u010CABRILO LAZAR", {0}},
+        {"\\u010CABRILO LJUBICA", {0}},
+        {"\\u010CABRILO SPASOJA", {0}},
+        {"\\u010CADE\\u0160 ZDENKA", {0}},
+        {"\\u010CADESKI BLAGOJE", {0}},
+        {"\\u010CADOVSKI VLADIMIR", {0}},
+        {"\\u010CAGLJEVI\\u0106 TOMA", {0}},
+        {"\\u010CAGOROVI\\u0106 VLADIMIR", {0}},
+        {"\\u010CAJA VANKA", {0}},
+        {"\\u010CAJI\\u0106 BOGOLJUB", {0}},
+        {"\\u010CAJI\\u0106 BORISLAV", {0}},
+        {"\\u010CAJI\\u0106 RADOSLAV", {0}},
+        {"\\u010CAK\\u0160IRAN MILADIN", {0}},
+        {"\\u010CAKAN EUGEN", {0}},
+        {"\\u010CAKAN EVGENIJE", {0}},
+        {"\\u010CAKAN IVAN", {0}},
+        {"\\u010CAKAN JULIJAN", {0}},
+        {"\\u010CAKAN MIHAJLO", {0}},
+        {"\\u010CAKAN STEVAN", {0}},
+        {"\\u010CAKAN VLADIMIR", {0}},
+        {"\\u010CAKAN VLADIMIR", {0}},
+        {"\\u010CAKAN VLADIMIR", {0}},
+        {"\\u010CAKARA ANA", {0}},
+        {"\\u010CAKAREVI\\u0106 MOMIR", {0}},
+        {"\\u010CAKAREVI\\u0106 NEDELJKO", {0}},
+        {"\\u010CAKI \\u0160ANDOR", {0}},
+        {"\\u010CAKI AMALIJA", {0}},
+        {"\\u010CAKI ANDRA\\u0160", {0}},
+        {"\\u010CAKI LADISLAV", {0}},
+        {"\\u010CAKI LAJO\\u0160", {0}},
+        {"\\u010CAKI LASLO", {0}},
     };
 
 
diff --git a/icu4c/source/test/cintltst/cbiapts.c b/icu4c/source/test/cintltst/cbiapts.c
index 92e6c1af4fd..8bf21aa2302 100644
--- a/icu4c/source/test/cintltst/cbiapts.c
+++ b/icu4c/source/test/cintltst/cbiapts.c
@@ -566,7 +566,7 @@ static void TestBreakIteratorRules() {
     ubrk_setText(bi,  uData, -1, &status);
 
     pos = ubrk_first(bi);
-    for (i=0; i<sizeof(breaks); i++) {
+    for (i=0; i<(int)sizeof(breaks); i++) {
         if (pos == i && breaks[i] != '*') {
             log_err("FAIL: unexpected break at position %d found\n", pos);
             break;
@@ -1031,7 +1031,7 @@ static const TestBISuppressionsItem testBISuppressionsItems[] = {
     { "de",             testSentenceSuppressionsDe, testSentFwdOffsetsDe,     testSentRevOffsetsDe     },
     { "es@ss=standard", testSentenceSuppressionsEs, testSentSuppFwdOffsetsEs, testSentSuppRevOffsetsEs },
     { "es",             testSentenceSuppressionsEs, testSentFwdOffsetsEs,     testSentRevOffsetsEs     },
-    { NULL, NULL, NULL }
+    { NULL, NULL, NULL, NULL }
 };
 
 static void TestBreakIteratorSuppressions(void) {
diff --git a/icu4c/source/test/cintltst/cbiditst.c b/icu4c/source/test/cintltst/cbiditst.c
index ead77b2b4a4..558f2a36363 100644
--- a/icu4c/source/test/cintltst/cbiditst.c
+++ b/icu4c/source/test/cintltst/cbiditst.c
@@ -398,9 +398,9 @@ static int u16ToPseudo(const int length, const UChar * input, char * output)
 static char * formatLevels(UBiDi *bidi, char *buffer) {
     UErrorCode ec = U_ZERO_ERROR;
     const UBiDiLevel* gotLevels = ubidi_getLevels(bidi, &ec);
-    int len = ubidi_getLength(bidi);
+    int32_t len = ubidi_getLength(bidi);
     char c;
-    int i, k;
+    int32_t i, k;
 
     if(U_FAILURE(ec)) {
         strcpy(buffer, "BAD LEVELS");
@@ -408,7 +408,7 @@ static char * formatLevels(UBiDi *bidi, char *buffer) {
     }
     for (i=0; i<len; i++) {
         k = gotLevels[i];
-        if (k >= sizeof(columns))
+        if (k >= (int32_t)sizeof(columns))
             c = '+';
         else
             c = columns[k];
@@ -4630,7 +4630,7 @@ static char * formatMap(const int32_t * map, int len, char * buffer)
         k = map[i];
         if (k < 0)
             c = '-';
-        else if (k >= sizeof(columns))
+        else if (k >= (int32_t)sizeof(columns))
             c = '+';
         else
             c = columns[k];
diff --git a/icu4c/source/test/cintltst/ccapitst.c b/icu4c/source/test/cintltst/ccapitst.c
index f74a810123b..73663f12459 100644
--- a/icu4c/source/test/cintltst/ccapitst.c
+++ b/icu4c/source/test/cintltst/ccapitst.c
@@ -1370,6 +1370,11 @@ static void TSCC_fromU(const void *context,
                         UConverterCallbackReason reason,
                         UErrorCode * err)
 {
+    // suppress compiler warnings about unused variables
+    (void)codeUnits;
+    (void)length;
+    (void)codePoint;
+
     TSCCContext *ctx = (TSCCContext*)context;
     UConverterFromUCallback junkFrom;
     
@@ -1416,6 +1421,10 @@ static void TSCC_toU(const void *context,
                         UConverterCallbackReason reason,
                         UErrorCode * err)
 {
+    // suppress compiler warnings about unused variables
+    (void)codeUnits;
+    (void)length;
+
     TSCCContext *ctx = (TSCCContext*)context;
     UConverterToUCallback junkFrom;
     
@@ -2197,10 +2206,10 @@ convertExStreaming(UConverter *srcCnv, UConverter *targetCnv,
         if(errorCode==U_BUFFER_OVERFLOW_ERROR) {
             /* continue converting another chunk */
             errorCode=U_ZERO_ERROR;
-            if(targetLength+chunkSize<=sizeof(targetBuffer)) {
+            if(targetLength+chunkSize<=(int32_t)sizeof(targetBuffer)) {
                 targetLimit=target+chunkSize;
             } else {
-                targetLimit=targetBuffer+sizeof(targetBuffer);
+                targetLimit=targetBuffer+(int32_t)sizeof(targetBuffer);
             }
         } else if(U_FAILURE(errorCode)) {
             /* failure */
@@ -2522,6 +2531,12 @@ static void testFromTruncatedUTF8(UConverter *utf8Cnv, UConverter *cnv, const ch
                                   char charUTF8[4], int32_t charUTF8Length,
                                   char char0[8], int32_t char0Length,
                                   char char1[8], int32_t char1Length) {
+    // suppress compiler warnings about unused variables
+    (void)char0;
+    (void)char0Length;
+    (void)char1;
+    (void)char1Length;
+
     char utf8[16];
     int32_t utf8Length;
 
diff --git a/icu4c/source/test/cintltst/citertst.c b/icu4c/source/test/cintltst/citertst.c
index c93c19c3685..5ffc714cfb3 100644
--- a/icu4c/source/test/cintltst/citertst.c
+++ b/icu4c/source/test/cintltst/citertst.c
@@ -1076,7 +1076,7 @@ static void TestDiscontiguos() {
         log_verbose("count %d\n", count);
 
         for (;;) {
-            uint32_t  ce;
+            int32_t  ce;
             UChar    *e = u_strchr(s, 0x20);
             if (e == 0) {
                 e = u_strchr(s, 0);
@@ -1088,7 +1088,7 @@ static void TestDiscontiguos() {
                 return;
             }
             while (ce != UCOL_NULLORDER) {
-                if (ce != (uint32_t)ucol_next(iter, &status) ||
+                if (ce != ucol_next(iter, &status) ||
                     U_FAILURE(status)) {
                     log_err("Discontiguos contraction test mismatch\n");
                     return;
diff --git a/icu4c/source/test/cintltst/cldrtest.c b/icu4c/source/test/cintltst/cldrtest.c
index 1c0f8e28ff0..87ae093c29c 100644
--- a/icu4c/source/test/cintltst/cldrtest.c
+++ b/icu4c/source/test/cintltst/cldrtest.c
@@ -454,6 +454,7 @@ static void
 testLCID(UResourceBundle *currentBundle,
          const char *localeName)
 {
+    (void)currentBundle; // suppress compiler warnings about unused variables
     UErrorCode status = U_ZERO_ERROR;
     uint32_t expectedLCID;
     char lcidStringC[64] = {0};
diff --git a/icu4c/source/test/cintltst/cloctst.c b/icu4c/source/test/cintltst/cloctst.c
index 3940a78a820..6c247e1f032 100644
--- a/icu4c/source/test/cintltst/cloctst.c
+++ b/icu4c/source/test/cintltst/cloctst.c
@@ -2004,7 +2004,7 @@ static void TestKeywordSet(void)
           resultLen = uloc_getKeywordValue(kwSetTestCases[i].x, kwSetTestCases[i].k, buffer, 1023, &status);
           if(U_FAILURE(status)) {
             log_err("Err on test case %d for getKeywordValue: got error %s\n", i, u_errorName(status));
-          } else if (resultLen != uprv_strlen(kwSetTestCases[i].v) || uprv_strcmp(buffer, kwSetTestCases[i].v) != 0) {
+          } else if (resultLen != (int32_t)uprv_strlen(kwSetTestCases[i].v) || uprv_strcmp(buffer, kwSetTestCases[i].v) != 0) {
             log_err("FAIL: #%d getKeywordValue: got %s (%d) expected %s (%d)\n", i, buffer, resultLen,
                     kwSetTestCases[i].v, uprv_strlen(kwSetTestCases[i].v));
           }
@@ -3397,7 +3397,7 @@ static void TestGetLocaleForLCID() {
             continue;
         }
         
-        if (length != uprv_strlen(temp2)) {
+        if (length != (int32_t)uprv_strlen(temp2)) {
             log_err("  returned length %d not correct for uloc_getLocaleForLCID(%#04x), expected %d\n", length, lcid, uprv_strlen(temp2));
         }
         
diff --git a/icu4c/source/test/cintltst/cmsccoll.c b/icu4c/source/test/cintltst/cmsccoll.c
index 8d7f6cb76e0..7ad23367e14 100644
--- a/icu4c/source/test/cintltst/cmsccoll.c
+++ b/icu4c/source/test/cintltst/cmsccoll.c
@@ -1454,14 +1454,14 @@ static void TestContraction(void) {
             UCollationElements *iter2 = ucol_openElements(coll,
                                                          &(testdata[i][j]),
                                                          1, &status);
-            uint32_t ce;
+            int32_t ce;
             if (U_FAILURE(status)) {
                 log_err("Collation iterator creation failed\n");
                 return;
             }
             ce = ucol_next(iter2, &status);
             while (ce != UCOL_NULLORDER) {
-                if ((uint32_t)ucol_next(iter1, &status) != ce) {
+                if (ucol_next(iter1, &status) != ce) {
                     log_err("Collation elements in contraction split does not match\n");
                     return;
                 }
diff --git a/icu4c/source/test/cintltst/cnormtst.c b/icu4c/source/test/cintltst/cnormtst.c
index fe7237abc81..8f31ad402d9 100644
--- a/icu4c/source/test/cintltst/cnormtst.c
+++ b/icu4c/source/test/cintltst/cnormtst.c
@@ -661,7 +661,7 @@ void TestCheckFCD()
   for (count = 0; count < 50; count ++)
   {
     int size = 0;
-    UBool testresult = UNORM_YES;
+    UNormalizationCheckResult testresult = UNORM_YES;
     UChar data[20];
     UChar norm[100];
     UChar nfd[100];
diff --git a/icu4c/source/test/cintltst/cnumtst.c b/icu4c/source/test/cintltst/cnumtst.c
index 1af478369f0..c4056424f1e 100644
--- a/icu4c/source/test/cintltst/cnumtst.c
+++ b/icu4c/source/test/cintltst/cnumtst.c
@@ -881,9 +881,9 @@ free(result);
             log_err("File %s, Line %d, (expected, acutal) =  (\"%s\", \"%s\")\n",
                     __FILE__, __LINE__, numFormatted, desta);
         }
-        if (strlen(numFormatted) != resultSize) {
+        if ((int32_t)strlen(numFormatted) != resultSize) {
             log_err("File %s, Line %d, (expected, actual) = (%d, %d)\n",
-                     __FILE__, __LINE__, strlen(numFormatted), resultSize);
+                     __FILE__, __LINE__, (int32_t)strlen(numFormatted), resultSize);
         }
 
         /* Format with a FieldPosition parameter */
@@ -922,9 +922,9 @@ free(result);
             log_verbose("File %s, Line %d, got expected = \"%s\"\n",
                     __FILE__, __LINE__, desta);
         }
-        if (strlen(parseExpected) != resultSize) {
+        if ((int32_t)strlen(parseExpected) != resultSize) {
             log_err("File %s, Line %d, (expected, actual) = (%d, %d)\n",
-                    __FILE__, __LINE__, strlen(parseExpected), resultSize);
+                    __FILE__, __LINE__, (int32_t)strlen(parseExpected), resultSize);
         }
 
         /* Parse with a parsePos parameter */
@@ -944,9 +944,9 @@ free(result);
             log_verbose("File %s, Line %d, got expected = \"%s\"\n",
                     __FILE__, __LINE__, desta);
         }
-        if (strlen(numFormatted) != parsePos) {
+        if ((int32_t)strlen(numFormatted) != parsePos) {
             log_err("File %s, Line %d, parsePos (expected, actual) = (\"%d\", \"%d\")\n",
-                    __FILE__, __LINE__, strlen(parseExpected), parsePos);
+                    __FILE__, __LINE__, (int32_t)strlen(parseExpected), parsePos);
         }
 
         unum_close(fmt);
diff --git a/icu4c/source/test/cintltst/creststn.c b/icu4c/source/test/cintltst/creststn.c
index a19723baf3d..e56c91da9d4 100644
--- a/icu4c/source/test/cintltst/creststn.c
+++ b/icu4c/source/test/cintltst/creststn.c
@@ -62,7 +62,7 @@ randul()
     }
     /* Assume rand has at least 12 bits of precision */
     
-    for (i=0; i<sizeof(l); ++i)
+    for (i=0; i<(int32_t)sizeof(l); ++i)
         ((char*)&l)[i] = (char)((rand() & 0x0FF0) >> 4);
     return l;
 }
@@ -2946,7 +2946,7 @@ tres_getString(const UResourceBundle *resB,
         }
 
         /* verify NUL-termination */
-        if((p8 != buffer8 || length8 < sizeof(buffer8)) && s8[length8] != 0) {
+        if((p8 != buffer8 || length8 < (int32_t)sizeof(buffer8)) && s8[length8] != 0) {
             log_err("ures_getUTF8String(%p, %ld, '%s') did not NUL-terminate\n",
                     resB, (long)idx, key);
         }
diff --git a/icu4c/source/test/cintltst/cstrcase.c b/icu4c/source/test/cintltst/cstrcase.c
index e526b54f4ab..7ea8424ac29 100644
--- a/icu4c/source/test/cintltst/cstrcase.c
+++ b/icu4c/source/test/cintltst/cstrcase.c
@@ -442,7 +442,7 @@ TestCaseFolding(void) {
 
     /* test simple case folding */
     p=simple;
-    for(i=0; i<sizeof(simple)/12; p+=3, ++i) {
+    for(i=0; i<(int32_t)sizeof(simple)/12; p+=3, ++i) {
         if(u_foldCase(p[0], U_FOLD_CASE_DEFAULT)!=p[1]) {
             log_err("error: u_foldCase(0x%04lx, default)=0x%04lx instead of 0x%04lx\n",
                     p[0], u_foldCase(p[0], U_FOLD_CASE_DEFAULT), p[1]);
diff --git a/icu4c/source/test/cintltst/cstrtest.c b/icu4c/source/test/cintltst/cstrtest.c
index 3903a523401..52142551376 100644
--- a/icu4c/source/test/cintltst/cstrtest.c
+++ b/icu4c/source/test/cintltst/cstrtest.c
@@ -271,7 +271,7 @@ TestInvariant() {
         log_err("uprv_isInvariantString(\"\\0\") failed\n");
     }
 
-    for(i=0; i<(sizeof(variantChars)-1); ++i) {
+    for(i=0; i<(int32_t)(sizeof(variantChars)-1); ++i) {
         if(uprv_isInvariantString(variantChars+i, 1)) {
             log_err("uprv_isInvariantString(variantChars[%d]) failed\n", i);
         }
diff --git a/icu4c/source/test/cintltst/cucdtst.c b/icu4c/source/test/cintltst/cucdtst.c
index 7a778638732..cd097320d9e 100644
--- a/icu4c/source/test/cintltst/cucdtst.c
+++ b/icu4c/source/test/cintltst/cucdtst.c
@@ -963,6 +963,7 @@ unicodeDataLineFn(void *context,
                   char *fields[][2], int32_t fieldCount,
                   UErrorCode *pErrorCode)
 {
+    (void)fieldCount; // suppress compiler warnings about unused variable
     char buffer[100];
     const char *d;
     char *end;
@@ -1022,7 +1023,7 @@ unicodeDataLineFn(void *context,
     /* get BiDi category, field 4 */
     *fields[4][1]=0;
     i=MakeDir(fields[4][0]);
-    if(i!=u_charDirection(c) || i!=u_getIntPropertyValue(c, UCHAR_BIDI_CLASS)) {
+    if(i!=(int32_t)u_charDirection(c) || i!=u_getIntPropertyValue(c, UCHAR_BIDI_CLASS)) {
         log_err("error: u_charDirection(U+%04lx)==%u instead of %u (%s)\n", c, u_charDirection(c), MakeDir(fields[4][0]), fields[4][0]);
     }
 
@@ -1217,6 +1218,8 @@ enumTypeRange(const void *context, UChar32 start, UChar32 limit, UCharCategory t
 
 static UBool U_CALLCONV
 enumDefaultsRange(const void *context, UChar32 start, UChar32 limit, UCharCategory type) {
+    (void)context; // suppress compiler warnings about unused variable
+
     /* default Bidi classes for unassigned code points, from the DerivedBidiClass.txt header */
     static const int32_t defaultBidi[][2]={ /* { limit, class } */
         { 0x0590, U_LEFT_TO_RIGHT },
@@ -1292,7 +1295,7 @@ enumDefaultsRange(const void *context, UChar32 start, UChar32 limit, UCharCatego
                     }
 
                     if( u_charDirection(c)!=shouldBeDir ||
-                        u_getIntPropertyValue(c, UCHAR_BIDI_CLASS)!=shouldBeDir
+                        (UCharDirection)u_getIntPropertyValue(c, UCHAR_BIDI_CLASS)!=shouldBeDir
                     ) {
                         log_err("error: u_charDirection(unassigned/PUA U+%04lx)=%s should be %s\n",
                             c, dirStrings[u_charDirection(c)], dirStrings[shouldBeDir]);
@@ -1651,28 +1654,28 @@ static const struct {
     uint32_t code;
     const char *name, *oldName, *extName, *alias;
 } names[]={
-    {0x0061, "LATIN SMALL LETTER A", "", "LATIN SMALL LETTER A"},
+    {0x0061, "LATIN SMALL LETTER A", "", "LATIN SMALL LETTER A", NULL},
     {0x01a2, "LATIN CAPITAL LETTER OI", "",
              "LATIN CAPITAL LETTER OI",
              "LATIN CAPITAL LETTER GHA"},
     {0x0284, "LATIN SMALL LETTER DOTLESS J WITH STROKE AND HOOK", "",
-             "LATIN SMALL LETTER DOTLESS J WITH STROKE AND HOOK" },
+             "LATIN SMALL LETTER DOTLESS J WITH STROKE AND HOOK", NULL},
     {0x0fd0, "TIBETAN MARK BSKA- SHOG GI MGO RGYAN", "",
              "TIBETAN MARK BSKA- SHOG GI MGO RGYAN",
              "TIBETAN MARK BKA- SHOG GI MGO RGYAN"},
-    {0x3401, "CJK UNIFIED IDEOGRAPH-3401", "", "CJK UNIFIED IDEOGRAPH-3401" },
-    {0x7fed, "CJK UNIFIED IDEOGRAPH-7FED", "", "CJK UNIFIED IDEOGRAPH-7FED" },
-    {0xac00, "HANGUL SYLLABLE GA", "", "HANGUL SYLLABLE GA" },
-    {0xd7a3, "HANGUL SYLLABLE HIH", "", "HANGUL SYLLABLE HIH" },
-    {0xd800, "", "", "<lead surrogate-D800>" },
-    {0xdc00, "", "", "<trail surrogate-DC00>" },
-    {0xff08, "FULLWIDTH LEFT PARENTHESIS", "", "FULLWIDTH LEFT PARENTHESIS" },
-    {0xffe5, "FULLWIDTH YEN SIGN", "", "FULLWIDTH YEN SIGN" },
-    {0xffff, "", "", "<noncharacter-FFFF>" },
+    {0x3401, "CJK UNIFIED IDEOGRAPH-3401", "", "CJK UNIFIED IDEOGRAPH-3401", NULL},
+    {0x7fed, "CJK UNIFIED IDEOGRAPH-7FED", "", "CJK UNIFIED IDEOGRAPH-7FED", NULL},
+    {0xac00, "HANGUL SYLLABLE GA", "", "HANGUL SYLLABLE GA", NULL},
+    {0xd7a3, "HANGUL SYLLABLE HIH", "", "HANGUL SYLLABLE HIH", NULL},
+    {0xd800, "", "", "<lead surrogate-D800>", NULL},
+    {0xdc00, "", "", "<trail surrogate-DC00>", NULL},
+    {0xff08, "FULLWIDTH LEFT PARENTHESIS", "", "FULLWIDTH LEFT PARENTHESIS", NULL},
+    {0xffe5, "FULLWIDTH YEN SIGN", "", "FULLWIDTH YEN SIGN", NULL},
+    {0xffff, "", "", "<noncharacter-FFFF>", NULL},
     {0x1d0c5, "BYZANTINE MUSICAL SYMBOL FHTORA SKLIRON CHROMA VASIS", "",
               "BYZANTINE MUSICAL SYMBOL FHTORA SKLIRON CHROMA VASIS",
               "BYZANTINE MUSICAL SYMBOL FTHORA SKLIRON CHROMA VASIS"},
-    {0x23456, "CJK UNIFIED IDEOGRAPH-23456", "", "CJK UNIFIED IDEOGRAPH-23456" }
+    {0x23456, "CJK UNIFIED IDEOGRAPH-23456", "", "CJK UNIFIED IDEOGRAPH-23456", NULL}
 };
 
 static UBool
@@ -1908,7 +1911,8 @@ TestCharNames() {
                 uset_add,
                 uset_addRange,
                 uset_addString,
-                NULL /* don't need remove() */
+                NULL, /* don't need remove() */
+                NULL  /* don't need removeRange() */
             };
             sa.set=set;
             uprv_getCharNameCharacters(&sa);
@@ -3418,6 +3422,8 @@ static void U_CALLCONV
 caseFoldingLineFn(void *context,
                   char *fields[][2], int32_t fieldCount,
                   UErrorCode *pErrorCode) {
+    (void)fieldCount; // suppress compiler warnings about unused variable
+
     CaseFoldingData *pData=(CaseFoldingData *)context;
     char *end;
     UChar full[32];
@@ -3525,7 +3531,7 @@ caseFoldingLineFn(void *context,
 
 static void
 TestCaseFolding() {
-    CaseFoldingData data={ NULL };
+    CaseFoldingData data={ NULL, 0, 0, {0}, 0, 0 };
     char *fields[3][2];
     UErrorCode errorCode;
 
diff --git a/icu4c/source/test/cintltst/currtest.c b/icu4c/source/test/cintltst/currtest.c
index 64df6727d07..514278ad588 100644
--- a/icu4c/source/test/cintltst/currtest.c
+++ b/icu4c/source/test/cintltst/currtest.c
@@ -122,7 +122,7 @@ static int32_t checkItemCount(uint32_t currencyType) {
     originalCount = uenum_count(en, &status);
     for (count=0;;count++) {
         const char *str = uenum_next(en, &len, &status);
-        if (str == NULL || len != expectedLen || strlen(str) != expectedLen) {
+        if (str == NULL || len != expectedLen || (int32_t)strlen(str) != expectedLen) {
             break;
         }
     }
diff --git a/icu4c/source/test/cintltst/custrtrn.c b/icu4c/source/test/cintltst/custrtrn.c
index 73a995ed91f..895022e6029 100644
--- a/icu4c/source/test/cintltst/custrtrn.c
+++ b/icu4c/source/test/cintltst/custrtrn.c
@@ -748,7 +748,7 @@ static void Test_UChar_UTF8_API(void){
                            withTrail16, u_strlen(withTrail16),
                            0xfffd, &numSubstitutions,
                            &err);
-        if(U_FAILURE(err) || u8DestLen!=uprv_strlen((const char *)withTrail8SubFFFD) ||
+        if(U_FAILURE(err) || u8DestLen!=(int32_t)uprv_strlen((const char *)withTrail8SubFFFD) ||
                              0!=uprv_memcmp((const char *)withTrail8SubFFFD, out8, u8DestLen+1) ||
                              numSubstitutions!=1) {
             log_err("error: u_strToUTF8WithSub(length) failed\n");
@@ -763,7 +763,7 @@ static void Test_UChar_UTF8_API(void){
                            withTrail16, -1,
                            0x1a, &numSubstitutions,
                            &err);
-        if(U_FAILURE(err) || u8DestLen!=uprv_strlen((const char *)withTrail8Sub1A) ||
+        if(U_FAILURE(err) || u8DestLen!=(int32_t)uprv_strlen((const char *)withTrail8Sub1A) ||
                              0!=uprv_memcmp((const char *)withTrail8Sub1A, out8, u8DestLen+1) ||
                              numSubstitutions!=1) {
             log_err("error: u_strToUTF8WithSub(NUL termination) failed\n");
@@ -778,7 +778,7 @@ static void Test_UChar_UTF8_API(void){
                            withTrail16, -1,
                            0xfffd, &numSubstitutions,
                            &err);
-        if(err!=U_BUFFER_OVERFLOW_ERROR || u8DestLen!=uprv_strlen((const char *)withTrail8SubFFFD) ||
+        if(err!=U_BUFFER_OVERFLOW_ERROR || u8DestLen!=(int32_t)uprv_strlen((const char *)withTrail8SubFFFD) ||
                                            numSubstitutions!=1) {
             log_err("error: u_strToUTF8WithSub(preflight/NUL termination) failed\n");
         }
diff --git a/icu4c/source/test/cintltst/hpmufn.c b/icu4c/source/test/cintltst/hpmufn.c
index 4cdf377e46d..f9ecc9b213c 100644
--- a/icu4c/source/test/cintltst/hpmufn.c
+++ b/icu4c/source/test/cintltst/hpmufn.c
@@ -83,6 +83,7 @@ int    gBlockCount = 0;
 const void  *gContext;
 
 static void * U_CALLCONV myMemAlloc(const void *context, size_t size) {
+    (void)context; // suppress compiler warnings about unused variable
     char *retPtr = (char *)malloc(size+sizeof(ctest_AlignedMemory));
     if (retPtr != NULL) {
         retPtr += sizeof(ctest_AlignedMemory);
@@ -92,6 +93,7 @@ static void * U_CALLCONV myMemAlloc(const void *context, size_t size) {
 }
 
 static void U_CALLCONV myMemFree(const void *context, void *mem) {
+    (void)context; // suppress compiler warnings about unused variable
     char *freePtr = (char *)mem;
     if (freePtr != NULL) {
         freePtr -= sizeof(ctest_AlignedMemory);
@@ -102,6 +104,7 @@ static void U_CALLCONV myMemFree(const void *context, void *mem) {
 
 
 static void * U_CALLCONV myMemRealloc(const void *context, void *mem, size_t size) {
+    (void)context; // suppress compiler warnings about unused variable
     char *p = (char *)mem;
     char *retPtr;
 
diff --git a/icu4c/source/test/cintltst/ncnvtst.c b/icu4c/source/test/cintltst/ncnvtst.c
index 255020a2e9c..143042fc11f 100644
--- a/icu4c/source/test/cintltst/ncnvtst.c
+++ b/icu4c/source/test/cintltst/ncnvtst.c
@@ -699,7 +699,7 @@ static void TestRegressionUTF8(){
         offset16 = 0;
         offset8 = 0;
         while(currCh <= UNICODE_LIMIT
-            && offset16 < (MAX_LENGTH/sizeof(UChar) - MAX_UTF16_LEN)
+            && offset16 < ((int32_t)(MAX_LENGTH/sizeof(UChar) - MAX_UTF16_LEN))
             && offset8 < (MAX_LENGTH - MAX_UTF8_LEN))
         {
             if (currCh == SURROGATE_HIGH_START) {
@@ -770,8 +770,8 @@ static void TestRegressionUTF32(){
         offset16 = 0;
         offset32 = 0;
         while(currCh <= UNICODE_LIMIT
-            && offset16 < (MAX_LENGTH/sizeof(UChar) - MAX_UTF16_LEN)
-            && offset32 < (MAX_LENGTH/sizeof(UChar32) - MAX_UTF32_LEN))
+            && offset16 < ((int32_t)(MAX_LENGTH/sizeof(UChar) - MAX_UTF16_LEN))
+            && offset32 < ((int32_t)(MAX_LENGTH/sizeof(UChar32) - MAX_UTF32_LEN)))
         {
             if (currCh == SURROGATE_HIGH_START) {
                 currCh = SURROGATE_LOW_END + 1; /* Skip surrogate range */
diff --git a/icu4c/source/test/cintltst/nucnvtst.c b/icu4c/source/test/cintltst/nucnvtst.c
index ba8d1313613..aa0a7c705c8 100644
--- a/icu4c/source/test/cintltst/nucnvtst.c
+++ b/icu4c/source/test/cintltst/nucnvtst.c
@@ -4581,6 +4581,10 @@ typedef struct {
 /* Callback for TestJitterbug6175, should only get called for empty segment errors */
 static void UCNV_TO_U_CALLBACK_EMPTYSEGMENT( const void *context, UConverterToUnicodeArgs *toArgs, const char* codeUnits,
                                              int32_t length, UConverterCallbackReason reason, UErrorCode * err ) {
+    // suppress compiler warnings about unused variables
+    (void)context;
+    (void)codeUnits;
+    (void)length;
     if (reason > UCNV_IRREGULAR) {
         return;
     }
diff --git a/icu4c/source/test/cintltst/putiltst.c b/icu4c/source/test/cintltst/putiltst.c
index 9bb5d186566..e455dd59441 100644
--- a/icu4c/source/test/cintltst/putiltst.c
+++ b/icu4c/source/test/cintltst/putiltst.c
@@ -36,7 +36,7 @@ static void TestSignedRightShiftIsArithmetic(void) {
     int32_t m=-1;
     int32_t x4=x>>4;
     int32_t m1=m>>1;
-    UBool signedRightShiftIsArithmetic= x4==0xffff5fff && m1==-1;
+    UBool signedRightShiftIsArithmetic= (x4==(int32_t)0xffff5fff && m1==-1);
     if(signedRightShiftIsArithmetic==U_SIGNED_RIGHT_SHIFT_IS_ARITHMETIC) {
         log_info("signed right shift is Arithmetic Shift Right: %d\n",
                  signedRightShiftIsArithmetic);
diff --git a/icu4c/source/test/cintltst/reapits.c b/icu4c/source/test/cintltst/reapits.c
index b71266938ce..a796e8c03c7 100644
--- a/icu4c/source/test/cintltst/reapits.c
+++ b/icu4c/source/test/cintltst/reapits.c
@@ -2253,10 +2253,16 @@ static void TestBug8421(void) {
 }
 
 static UBool U_CALLCONV FindCallback(const void* context , int64_t matchIndex) {
+    // suppress compiler warnings about unused variables
+    (void)context;
+    (void)matchIndex;
     return FALSE;
 }
 
 static UBool U_CALLCONV MatchCallback(const void *context, int32_t steps) {
+    // suppress compiler warnings about unused variables
+    (void)context;
+    (void)steps;
     return FALSE;
 }
 
diff --git a/icu4c/source/test/cintltst/sorttest.c b/icu4c/source/test/cintltst/sorttest.c
index 79d17730223..d4d8136e528 100644
--- a/icu4c/source/test/cintltst/sorttest.c
+++ b/icu4c/source/test/cintltst/sorttest.c
@@ -109,6 +109,7 @@ typedef struct Line {
 
 static void
 printLines(const Line *lines) {
+    (void)lines; // suppress compiler warnings about unused variable
 #if 0
     int32_t i, j;
     for(i=0; i<NUM_LINES; ++i) {
diff --git a/icu4c/source/test/cintltst/spreptst.c b/icu4c/source/test/cintltst/spreptst.c
index 2d8e34c74a7..bd0c93ebc07 100644
--- a/icu4c/source/test/cintltst/spreptst.c
+++ b/icu4c/source/test/cintltst/spreptst.c
@@ -295,6 +295,7 @@ static int32_t
 unescapeData(const char* src, int32_t srcLen, 
              char* dest, int32_t destCapacity, 
              UErrorCode* status){
+    (void)srcLen; // suppress compiler warnings about unused variable
 
     UChar b1Stack[MAX_BUFFER_SIZE];
     int32_t b1Capacity = MAX_BUFFER_SIZE,
diff --git a/icu4c/source/test/cintltst/sprpdata.c b/icu4c/source/test/cintltst/sprpdata.c
index b3ac72d79c1..af249a0aa6b 100644
--- a/icu4c/source/test/cintltst/sprpdata.c
+++ b/icu4c/source/test/cintltst/sprpdata.c
@@ -49,6 +49,7 @@ static void U_CALLCONV
 strprepProfileLineFn(void *context,
               char *fields[][2], int32_t fieldCount,
               UErrorCode *pErrorCode) {
+    (void)fieldCount; // suppress compiler warnings about unused variable
     uint32_t mapping[40];
     char *end, *map;
     uint32_t code;
@@ -273,7 +274,7 @@ compareFlagsForRange(UStringPrepProfile* data,
 
 void
 doStringPrepTest(const char* binFileName, const char* txtFileName, int32_t options, UErrorCode* errorCode){
-
+    (void)options; // suppress compiler warnings about unused variable
     const char *testdatapath = loadTestData(errorCode);
     const char *srcdatapath = NULL;
     const char *relativepath = NULL;
diff --git a/icu4c/source/test/cintltst/tracetst.c b/icu4c/source/test/cintltst/tracetst.c
index 94d10046aeb..28b9b04e060 100644
--- a/icu4c/source/test/cintltst/tracetst.c
+++ b/icu4c/source/test/cintltst/tracetst.c
@@ -77,7 +77,7 @@ static void test_format(const char *format, int32_t bufCap, int32_t indent,
     char  expectedResult[300];
 
     /* check that local buffers are big enough for the test case */
-    if (sizeof(buf) <= bufCap) {
+    if ((int32_t)sizeof(buf) <= bufCap) {
         log_err("At file:line %s:%d, requested bufCap too large.\n");
         return;
     }
@@ -115,6 +115,7 @@ static UBool  gFnNameError   = FALSE;
 static UBool  gFnFormatError = FALSE;
 
 static void U_CALLCONV testTraceEntry(const void *context, int32_t fnNumber) {
+    (void)context; // suppress compiler warnings about unused variable
     const char *fnName;
     const char *bogusFnName;
 
@@ -132,6 +133,7 @@ static void U_CALLCONV testTraceEntry(const void *context, int32_t fnNumber) {
 
 static void U_CALLCONV testTraceExit(const void *context, int32_t fnNumber,
                    const char *fmt, va_list args) {
+    (void)context; // suppress compiler warnings about unused variable
     char        buf[1000];
     const char *fnName;
     const char *bogusFnName;
@@ -158,6 +160,9 @@ static void U_CALLCONV testTraceExit(const void *context, int32_t fnNumber,
 
 static void U_CALLCONV testTraceData(const void *context, int32_t fnNumber, int32_t level,
                    const char *fmt, va_list args) {
+    // suppress compiler warnings about unused variables
+    (void)context;
+    (void)level;
     char        buf[1000];
     const char *fnName;
     const char *bogusFnName;
diff --git a/icu4c/source/test/cintltst/trie2test.c b/icu4c/source/test/cintltst/trie2test.c
index aa7e7c64671..845e3fe30c2 100644
--- a/icu4c/source/test/cintltst/trie2test.c
+++ b/icu4c/source/test/cintltst/trie2test.c
@@ -75,6 +75,7 @@ getSpecialValues(const CheckRange checkRanges[], int32_t countCheckRanges,
 /* utrie2_enum() callback, modifies a value */
 static uint32_t U_CALLCONV
 testEnumValue(const void *context, uint32_t value) {
+    (void)context; // suppress compiler warnings about unused variable
     return value^0x5555;
 }
 
@@ -98,6 +99,7 @@ static void
 testTrieEnum(const char *testName,
              const UTrie2 *trie,
              const CheckRange checkRanges[], int32_t countCheckRanges) {
+    (void)testName; // suppress compiler warnings about unused variable
     /* skip over special values */
     while(countCheckRanges>0 && checkRanges[0].limit<=0) {
         ++checkRanges;
@@ -430,7 +432,7 @@ testTrieUTF8(const char *testName,
             values[countValues++]=value;
         }
         /* write an illegal byte sequence */
-        if(i8<sizeof(illegal)) {
+        if(i8<(int32_t)sizeof(illegal)) {
             U8_FWD_1(illegal, i8, sizeof(illegal));
             while(prev8<i8) {
                 s[length++]=illegal[prev8++];
@@ -439,7 +441,7 @@ testTrieUTF8(const char *testName,
         }
     }
     /* write the remaining illegal byte sequences */
-    while(i8<sizeof(illegal)) {
+    while(i8<(int32_t)sizeof(illegal)) {
         U8_FWD_1(illegal, i8, sizeof(illegal));
         while(prev8<i8) {
             s[length++]=illegal[prev8++];
diff --git a/icu4c/source/test/cintltst/trietest.c b/icu4c/source/test/cintltst/trietest.c
index bf770a10fa4..b70bf539d50 100644
--- a/icu4c/source/test/cintltst/trietest.c
+++ b/icu4c/source/test/cintltst/trietest.c
@@ -120,6 +120,7 @@ _testFoldingOffset16(uint32_t data) {
 
 static uint32_t U_CALLCONV
 _testEnumValue(const void *context, uint32_t value) {
+    (void)context; // suppress compiler warnings about unused variable
     return value^0x5555;
 }
 
@@ -815,6 +816,7 @@ TrieTest(void) {
 
 static int32_t U_CALLCONV
 dummyGetFoldingOffset(uint32_t data) {
+    (void)data; // suppress compiler warnings about unused variable
     return -1; /* never get non-initialValue data for supplementary code points */
 }
 
diff --git a/icu4c/source/test/cintltst/ucnvseltst.c b/icu4c/source/test/cintltst/ucnvseltst.c
index d990ea4f8f4..bad4e805554 100644
--- a/icu4c/source/test/cintltst/ucnvseltst.c
+++ b/icu4c/source/test/cintltst/ucnvseltst.c
@@ -240,7 +240,7 @@ text_open(TestText *tt) {
     fclose(f);
     return FALSE;
   }
-  if (length != fread(tt->text, 1, length, f)) {
+  if (length != (int32_t)fread(tt->text, 1, length, f)) {
     log_err("error reading %ld bytes from test text file\n", (long)length);
     length = 0;
     uprv_free(tt->text);
diff --git a/icu4c/source/test/cintltst/ucptrietest.c b/icu4c/source/test/cintltst/ucptrietest.c
index 299ef900b83..9d2e489f97f 100644
--- a/icu4c/source/test/cintltst/ucptrietest.c
+++ b/icu4c/source/test/cintltst/ucptrietest.c
@@ -67,6 +67,7 @@ getSpecialValues(const CheckRange checkRanges[], int32_t countCheckRanges,
 /* ucptrie_enum() callback, modifies a value */
 static uint32_t U_CALLCONV
 testFilter(const void *context, uint32_t value) {
+    (void)context; // suppress compiler warnings about unused variable
     return value ^ 0x5555;
 }
 
@@ -568,7 +569,7 @@ testTrieUTF8(const char *testName,
             values[countValues++]=value;
         }
         /* write an illegal byte sequence */
-        if(i8<sizeof(illegal)) {
+        if(i8<(int32_t)sizeof(illegal)) {
             U8_FWD_1(illegal, i8, sizeof(illegal));
             while(prev8<i8) {
                 s[length++]=illegal[prev8++];
@@ -577,7 +578,7 @@ testTrieUTF8(const char *testName,
         }
     }
     /* write the remaining illegal byte sequences */
-    while(i8<sizeof(illegal)) {
+    while(i8<(int32_t)sizeof(illegal)) {
         U8_FWD_1(illegal, i8, sizeof(illegal));
         while(prev8<i8) {
             s[length++]=illegal[prev8++];
diff --git a/icu4c/source/test/cintltst/udatatst.c b/icu4c/source/test/cintltst/udatatst.c
index d8d6ebd3534..5c57d5aa437 100644
--- a/icu4c/source/test/cintltst/udatatst.c
+++ b/icu4c/source/test/cintltst/udatatst.c
@@ -558,6 +558,7 @@ static UBool U_CALLCONV
 isAcceptable1(void *context,
              const char *type, const char *name,
              const UDataInfo *pInfo) {
+    (void)context; // suppress compiler warnings about unused variable
 
     if( pInfo->size>=20 &&
         pInfo->isBigEndian==U_IS_BIG_ENDIAN &&
@@ -590,6 +591,7 @@ static UBool U_CALLCONV
 isAcceptable2(void *context, 
              const char *type, const char *name,
       const UDataInfo *pInfo){
+    (void)context; // suppress compiler warnings about unused variable
     UVersionInfo unicodeVersion;
 
     u_getUnicodeVersion(unicodeVersion);
@@ -618,6 +620,7 @@ static UBool U_CALLCONV
 isAcceptable3(void *context, 
              const char *type, const char *name,
              const UDataInfo *pInfo){
+    (void)context; // suppress compiler warnings about unused variable
 
     if( pInfo->size>=20 &&
         pInfo->isBigEndian==U_IS_BIG_ENDIAN &&
@@ -1233,7 +1236,8 @@ static const struct {
 
     {0x31, 0x31, 0x31, 0x31},     /* dataFormat="1111" */
     {0, 0, 0, 0},                 /* formatVersion */
-    {0, 0, 0, 0}}                 /* dataVersion */
+    {0, 0, 0, 0}},                /* dataVersion */
+    0
 };
 #endif
 
@@ -1395,6 +1399,7 @@ static const struct {
 
 static void U_CALLCONV
 printError(void *context, const char *fmt, va_list args) {
+    (void)context; // suppress compiler warnings about unused variable
     vlog_info("[swap] ", fmt, args);
     log_err("\n");  /* Register error */
 }
diff --git a/icu4c/source/test/cintltst/uenumtst.c b/icu4c/source/test/cintltst/uenumtst.c
index 39e2494975f..8ce05e9b9cb 100644
--- a/icu4c/source/test/cintltst/uenumtst.c
+++ b/icu4c/source/test/cintltst/uenumtst.c
@@ -59,11 +59,13 @@ chArrayClose(UEnumeration *en) {
 
 static int32_t U_CALLCONV
 chArrayCount(UEnumeration *en, UErrorCode *status) {
+    (void)status; // suppress compiler warnings about unused variable
     return cont->maxIndex;
 }
 
 static const UChar* U_CALLCONV 
 chArrayUNext(UEnumeration *en, int32_t *resultLength, UErrorCode *status) {
+    (void)status; // suppress compiler warnings about unused variable
     if(cont->currIndex >= cont->maxIndex) {
         return NULL;
     }
@@ -81,6 +83,7 @@ chArrayUNext(UEnumeration *en, int32_t *resultLength, UErrorCode *status) {
 
 static const char* U_CALLCONV
 chArrayNext(UEnumeration *en, int32_t *resultLength, UErrorCode *status) {
+    (void)status; // suppress compiler warnings about unused variable
     if(cont->currIndex >= cont->maxIndex) {
         return NULL;
     }
@@ -93,6 +96,7 @@ chArrayNext(UEnumeration *en, int32_t *resultLength, UErrorCode *status) {
 
 static void U_CALLCONV
 chArrayReset(UEnumeration *en, UErrorCode *status) {
+    (void)status; // suppress compiler warnings about unused variable
     cont->currIndex = 0;
 }
 
@@ -160,11 +164,13 @@ uchArrayClose(UEnumeration *en) {
 
 static int32_t U_CALLCONV
 uchArrayCount(UEnumeration *en, UErrorCode *status) {
+    (void)status; // suppress compiler warnings about unused variable
     return ucont->maxIndex;
 }
 
 static const UChar* U_CALLCONV
 uchArrayUNext(UEnumeration *en, int32_t *resultLength, UErrorCode *status) {
+    (void)status; // suppress compiler warnings about unused variable
     if(ucont->currIndex >= ucont->maxIndex) {
         return NULL;
     }
@@ -177,6 +183,7 @@ uchArrayUNext(UEnumeration *en, int32_t *resultLength, UErrorCode *status) {
 
 static void U_CALLCONV
 uchArrayReset(UEnumeration *en, UErrorCode *status) {
+    (void)status; // suppress compiler warnings about unused variable
     ucont->currIndex = 0;
 }
 
@@ -343,7 +350,7 @@ static void verifyEnumeration(int line, UEnumeration *u, const char * const * co
         log_verbose("%s:%d: OK: string #%d got '%s'\n", __FILE__, line, i, c);
       }
       
-      if(len!=strlen(compareToChar[i])) {
+      if(len!=(int32_t)strlen(compareToChar[i])) {
         log_err("%s:%d: FAIL: string #%d expected len %d got %d\n", __FILE__, line, i, strlen(compareToChar[i]), len);
       } else {
         log_verbose("%s:%d: OK: string #%d got len %d\n", __FILE__, line, i, len);
@@ -380,7 +387,7 @@ static void verifyEnumeration(int line, UEnumeration *u, const char * const * co
         log_verbose("%s:%d: OK: ustring #%d got '%s'\n", __FILE__, line, i, compareToChar[i]);
       }
       
-      if(len!=strlen(compareToChar[i])) {
+      if(len!=(int32_t)strlen(compareToChar[i])) {
         log_err("%s:%d: FAIL: ustring #%d expected len %d got %d\n", __FILE__, line, i, strlen(compareToChar[i]), len);
       } else {
         log_verbose("%s:%d: OK: ustring #%d got len %d\n", __FILE__, line, i, len);
diff --git a/icu4c/source/test/cintltst/uformattedvaluetst.c b/icu4c/source/test/cintltst/uformattedvaluetst.c
index a5e37866459..392d76e3404 100644
--- a/icu4c/source/test/cintltst/uformattedvaluetst.c
+++ b/icu4c/source/test/cintltst/uformattedvaluetst.c
@@ -213,6 +213,7 @@ void checkFormattedValue(
         UFieldCategory expectedCategory,
         const UFieldPosition* expectedFieldPositions,
         int32_t expectedFieldPositionsLength) {
+    (void)expectedFieldPositionsLength; // suppress compiler warnings about unused variable
     UErrorCode ec = U_ZERO_ERROR;
     checkFormattedValueString(message, fv, expectedString, &ec);
     if (U_FAILURE(ec)) { return; }
@@ -244,6 +245,7 @@ void checkMixedFormattedValue(
         const UChar* expectedString,
         const UFieldPositionWithCategory* expectedFieldPositions,
         int32_t length) {
+    (void)length; // suppress compiler warnings about unused variable
     UErrorCode ec = U_ZERO_ERROR;
     checkFormattedValueString(message, fv, expectedString, &ec);
     if (U_FAILURE(ec)) { return; }
diff --git a/icu4c/source/test/cintltst/unumberformattertst.c b/icu4c/source/test/cintltst/unumberformattertst.c
index b4506500098..2e296f033f9 100644
--- a/icu4c/source/test/cintltst/unumberformattertst.c
+++ b/icu4c/source/test/cintltst/unumberformattertst.c
@@ -96,7 +96,7 @@ static void TestSkeletonFormatToFields() {
     if (assertSuccessCheck("unumf_formatInt() failed", &ec, TRUE)) {
 
         // field position test:
-        UFieldPosition ufpos = {UNUM_DECIMAL_SEPARATOR_FIELD};
+        UFieldPosition ufpos = {UNUM_DECIMAL_SEPARATOR_FIELD, 0, 0};
         unumf_resultNextFieldPosition(uresult, &ufpos, &ec);
         assertIntEquals("Field position should be correct", 14, ufpos.beginIndex);
         assertIntEquals("Field position should be correct", 15, ufpos.endIndex);
@@ -118,7 +118,7 @@ static void TestSkeletonFormatToFields() {
                 {UNUM_MEASURE_UNIT_FIELD, 18, 19}
             };
             UFieldPosition actual;
-            for (int32_t i = 0; i < sizeof(expectedFields) / sizeof(*expectedFields); i++) {
+            for (int32_t i = 0; i < (int32_t)(sizeof(expectedFields) / sizeof(*expectedFields)); i++) {
                 // Iterate using the UFieldPosition to hold state...
                 UFieldPosition expected = expectedFields[i];
                 actual.field = ufieldpositer_next(ufpositer, &actual.beginIndex, &actual.endIndex);
diff --git a/icu4c/source/test/cintltst/usrchtst.c b/icu4c/source/test/cintltst/usrchtst.c
index d9c306646d1..320001a9fd9 100644
--- a/icu4c/source/test/cintltst/usrchtst.c
+++ b/icu4c/source/test/cintltst/usrchtst.c
@@ -362,7 +362,7 @@ static UBool assertEqualWithUStringSearch(      UStringSearch *strsrch,
         matchlength = search.size[count];
         usearch_next(strsrch, &status);
         if (matchindex != usearch_getMatchedStart(strsrch) || 
-            matchlength != (uint32_t)usearch_getMatchedLength(strsrch)) {
+            matchlength != usearch_getMatchedLength(strsrch)) {
             char *str = toCharString(usearch_getText(strsrch, &textlength));
             log_err("Text: %s\n", str);
             str = toCharString(usearch_getPattern(strsrch, &textlength));
@@ -375,7 +375,7 @@ static UBool assertEqualWithUStringSearch(      UStringSearch *strsrch,
         count ++;
         
         if (usearch_getMatchedText(strsrch, matchtext, 128, &status) !=
-            (int32_t) matchlength || U_FAILURE(status) ||
+             matchlength || U_FAILURE(status) ||
             memcmp(matchtext, 
                    usearch_getText(strsrch, &textlength) + matchindex,
                    matchlength * sizeof(UChar)) != 0) {
@@ -404,7 +404,7 @@ static UBool assertEqualWithUStringSearch(      UStringSearch *strsrch,
         matchlength = search.size[count];
         usearch_previous(strsrch, &status);
         if (matchindex != usearch_getMatchedStart(strsrch) || 
-            matchlength != (uint32_t)usearch_getMatchedLength(strsrch)) {
+            matchlength != usearch_getMatchedLength(strsrch)) {
             char *str = toCharString(usearch_getText(strsrch, &textlength));
             log_err("Text: %s\n", str);
             str = toCharString(usearch_getPattern(strsrch, &textlength));
@@ -416,7 +416,7 @@ static UBool assertEqualWithUStringSearch(      UStringSearch *strsrch,
         }
         
         if (usearch_getMatchedText(strsrch, matchtext, 128, &status) !=
-            (int32_t) matchlength || U_FAILURE(status) ||
+             matchlength || U_FAILURE(status) ||
             memcmp(matchtext, 
                    usearch_getText(strsrch, &textlength) + matchindex,
                    matchlength * sizeof(UChar)) != 0) {
@@ -1255,10 +1255,10 @@ static void TestGetSetOffset(void)
         ucol_setStrength(usearch_getCollator(strsrch), search.strength);
         usearch_reset(strsrch);
         while (U_SUCCESS(status) && matchindex >= 0) {
-            uint32_t matchlength = search.size[count];
+            int32_t matchlength = search.size[count];
             usearch_next(strsrch, &status);
             if (matchindex != usearch_getMatchedStart(strsrch) || 
-                matchlength != (uint32_t)usearch_getMatchedLength(strsrch)) {
+                matchlength != usearch_getMatchedLength(strsrch)) {
                 char *str = toCharString(usearch_getText(strsrch, 
                                                          &textlength));
                 log_err("Text: %s\n", str);
@@ -1272,7 +1272,7 @@ static void TestGetSetOffset(void)
             usearch_setOffset(strsrch, matchindex + matchlength, &status);
             usearch_previous(strsrch, &status);
             if (matchindex != usearch_getMatchedStart(strsrch) || 
-                matchlength != (uint32_t)usearch_getMatchedLength(strsrch)) {
+                matchlength != usearch_getMatchedLength(strsrch)) {
                 char *str = toCharString(usearch_getText(strsrch, 
                                                          &textlength));
                 log_err("Text: %s\n", str);
@@ -1459,23 +1459,23 @@ static void TestGetMatch(void)
         }
         status = U_ZERO_ERROR;
         if (usearch_getMatchedText(strsrch, NULL, 0, &status) != 
-            (int32_t)matchlength || U_SUCCESS(status)){
+            matchlength || U_SUCCESS(status)){
             log_err("Error pre-flighting match length\n");
         }
         status = U_ZERO_ERROR;
         if (usearch_getMatchedText(strsrch, matchtext, 0, &status) != 
-            (int32_t)matchlength || U_SUCCESS(status)){
+            matchlength || U_SUCCESS(status)){
             log_err("Error getting match text with buffer size 0\n");
         }
         status = U_ZERO_ERROR;
         if (usearch_getMatchedText(strsrch, matchtext, matchlength, &status) 
-            != (int32_t)matchlength || matchtext[matchlength - 1] == 0 ||
+            != matchlength || matchtext[matchlength - 1] == 0 ||
             U_FAILURE(status)){
             log_err("Error getting match text with exact size\n");
         }
         status = U_ZERO_ERROR;
         if (usearch_getMatchedText(strsrch, matchtext, 128, &status) !=
-            (int32_t) matchlength || U_FAILURE(status) ||
+            matchlength || U_FAILURE(status) ||
             memcmp(matchtext, 
                    usearch_getText(strsrch, &textlength) + matchindex,
                    matchlength * sizeof(UChar)) != 0 ||
@@ -2313,10 +2313,10 @@ static void TestGetSetOffsetCanonical(void)
         usearch_setText(strsrch, text, -1, &status);
         usearch_setPattern(strsrch, pattern, -1, &status);
         while (U_SUCCESS(status) && matchindex >= 0) {
-            uint32_t matchlength = search.size[count];
+            int32_t matchlength = search.size[count];
             usearch_next(strsrch, &status);
             if (matchindex != usearch_getMatchedStart(strsrch) || 
-                matchlength != (uint32_t)usearch_getMatchedLength(strsrch)) {
+                matchlength != usearch_getMatchedLength(strsrch)) {
                 char *str = toCharString(usearch_getText(strsrch, 
                                                          &textlength));
                 log_err("Text: %s\n", str);
diff --git a/icu4c/source/test/cintltst/utf8tst.c b/icu4c/source/test/cintltst/utf8tst.c
index 9f4109c2078..4c1c71c1428 100644
--- a/icu4c/source/test/cintltst/utf8tst.c
+++ b/icu4c/source/test/cintltst/utf8tst.c
@@ -884,13 +884,13 @@ static void TestSetChar() {
         }
 #if !U_HIDE_OBSOLETE_UTF_OLD_H
         setOffset=offset;
-        UTF8_SET_CHAR_LIMIT_SAFE(input,0, setOffset, sizeof(input));
+        UTF8_SET_CHAR_LIMIT_SAFE(input,0, setOffset, (int32_t)sizeof(input));
         if(setOffset != limit_safe[i]){
             log_err("ERROR: UTF8_SET_CHAR_LIMIT_SAFE failed for offset=%ld. Expected:%ld Got:%ld\n", offset, limit_safe[i], setOffset);
         }
 #endif
         setOffset=offset;
-        U8_SET_CP_LIMIT(input,0, setOffset, sizeof(input));
+        U8_SET_CP_LIMIT(input,0, setOffset, (int32_t)sizeof(input));
         if(setOffset != limit_safe[i]){
             log_err("ERROR: U8_SET_CP_LIMIT failed for offset=%ld. Expected:%ld Got:%ld\n", offset, limit_safe[i], setOffset);
         }
diff --git a/icu4c/source/test/cintltst/utmstest.c b/icu4c/source/test/cintltst/utmstest.c
index 122ba30034c..c25b79aad91 100644
--- a/icu4c/source/test/cintltst/utmstest.c
+++ b/icu4c/source/test/cintltst/utmstest.c
@@ -52,7 +52,7 @@ static uint64_t randomInt64(void)
     }
 
     /* Assume rand has at least 12 bits of precision */
-    for (i = 0; i < sizeof(ran); i += 1) {
+    for (i = 0; i < (int32_t)sizeof(ran); i += 1) {
         ((char*)&ran)[i] = (char)((rand() & 0x0FF0) >> 4);
     }
 
diff --git a/icu4c/source/test/intltest/apicoll.cpp b/icu4c/source/test/intltest/apicoll.cpp
index b08211a1d96..37ce414bb7d 100644
--- a/icu4c/source/test/intltest/apicoll.cpp
+++ b/icu4c/source/test/intltest/apicoll.cpp
@@ -2048,9 +2048,9 @@ public:
     virtual UBool operator==(const Collator& other) const;
     // Collator::operator!= calls !Collator::operator== which works for all subclasses.
     virtual void setLocales(const Locale& requestedLocale, const Locale& validLocale, const Locale& actualLocale);
-    TestCollator() : Collator() {};
+    TestCollator() : Collator() {}
     TestCollator(UCollationStrength collationStrength, 
-           UNormalizationMode decompositionMode) : Collator(collationStrength, decompositionMode) {};
+           UNormalizationMode decompositionMode) : Collator(collationStrength, decompositionMode) {}
 };
 
 inline UBool TestCollator::operator==(const Collator& other) const {
diff --git a/icu4c/source/test/intltest/citrtest.cpp b/icu4c/source/test/intltest/citrtest.cpp
index 84a2b69ee09..375b3c129c6 100644
--- a/icu4c/source/test/intltest/citrtest.cpp
+++ b/icu4c/source/test/intltest/citrtest.cpp
@@ -36,7 +36,7 @@ public:
 
     }
 
-    virtual ~SCharacterIterator(){};
+    virtual ~SCharacterIterator(){}
 
                                 
     void setText(const UnicodeString& newText){
@@ -65,19 +65,19 @@ public:
     }
     virtual UChar nextPostInc(void){ return text.charAt(pos++);}
     virtual UChar32 next32PostInc(void){return text.char32At(pos++);}
-    virtual UBool hasNext() { return TRUE;};
-    virtual UChar first(){return DONE;};
-    virtual UChar32 first32(){return DONE;};
-    virtual UChar last(){return DONE;};
-    virtual UChar32 last32(){return DONE;};
-    virtual UChar setIndex(int32_t /*pos*/){return DONE;};
-    virtual UChar32 setIndex32(int32_t /*pos*/){return DONE;};
-    virtual UChar current() const{return DONE;};
-    virtual UChar32 current32() const{return DONE;};
-    virtual UChar next(){return DONE;};
-    virtual UChar32 next32(){return DONE;};
-    virtual UChar previous(){return DONE;};
-    virtual UChar32 previous32(){return DONE;};
+    virtual UBool hasNext() { return TRUE;}
+    virtual UChar first(){return DONE;}
+    virtual UChar32 first32(){return DONE;}
+    virtual UChar last(){return DONE;}
+    virtual UChar32 last32(){return DONE;}
+    virtual UChar setIndex(int32_t /*pos*/){return DONE;}
+    virtual UChar32 setIndex32(int32_t /*pos*/){return DONE;}
+    virtual UChar current() const{return DONE;}
+    virtual UChar32 current32() const{return DONE;}
+    virtual UChar next(){return DONE;}
+    virtual UChar32 next32(){return DONE;}
+    virtual UChar previous(){return DONE;}
+    virtual UChar32 previous32(){return DONE;}
     virtual int32_t move(int32_t delta,CharacterIterator::EOrigin origin){    
         switch(origin) {
         case kStart:
@@ -100,7 +100,7 @@ public:
         }
 
         return pos;
-    };
+    }
     virtual int32_t move32(int32_t delta, CharacterIterator::EOrigin origin){    
         switch(origin) {
         case kStart:
@@ -127,8 +127,8 @@ public:
         }
 
         return pos;
-    };
-    virtual UBool hasPrevious() {return TRUE;};
+    }
+    virtual UBool hasPrevious() {return TRUE;}
 
   SCharacterIterator&  operator=(const SCharacterIterator&    that){
      text = that.text;
diff --git a/icu4c/source/test/intltest/dcfmtest.cpp b/icu4c/source/test/intltest/dcfmtest.cpp
index 4b41663bb85..a07f1580bb3 100644
--- a/icu4c/source/test/intltest/dcfmtest.cpp
+++ b/icu4c/source/test/intltest/dcfmtest.cpp
@@ -106,7 +106,7 @@ if (status!=errcode) {dataerrln("DecimalFormatTest failure at line %d.  Expected
 class InvariantStringPiece: public StringPiece {
   public:
     InvariantStringPiece(const UnicodeString &s);
-    ~InvariantStringPiece() {};
+    ~InvariantStringPiece() {}
   private:
     MaybeStackArray<char, 20>  buf;
 };
@@ -130,7 +130,7 @@ InvariantStringPiece::InvariantStringPiece(const UnicodeString &s) {
 class UnicodeStringPiece: public StringPiece {
   public:
     UnicodeStringPiece(const UnicodeString &s);
-    ~UnicodeStringPiece() {};
+    ~UnicodeStringPiece() {}
   private:
     MaybeStackArray<char, 20>  buf;
 };
diff --git a/icu4c/source/test/intltest/numfmtst.cpp b/icu4c/source/test/intltest/numfmtst.cpp
index 8e7befe2dea..6e1617a358c 100644
--- a/icu4c/source/test/intltest/numfmtst.cpp
+++ b/icu4c/source/test/intltest/numfmtst.cpp
@@ -288,7 +288,7 @@ NumberFormatTest::TestAPI(void)
 
 class StubNumberFormat :public NumberFormat{
 public:
-    StubNumberFormat(){};
+    StubNumberFormat(){}
     virtual UnicodeString& format(double ,UnicodeString& appendTo,FieldPosition& ) const {
         return appendTo;
     }
diff --git a/icu4c/source/test/intltest/plurults.cpp b/icu4c/source/test/intltest/plurults.cpp
index 1afb9c445f3..ba5817e811a 100644
--- a/icu4c/source/test/intltest/plurults.cpp
+++ b/icu4c/source/test/intltest/plurults.cpp
@@ -69,9 +69,9 @@ class US {
     US(const UnicodeString &us) {
        int32_t bufLen = us.extract((int32_t)0, us.length(), (char *)NULL, (uint32_t)0) + 1;
        buf = (char *)uprv_malloc(bufLen);
-       us.extract(0, us.length(), buf, bufLen); };
-    const char *cstr() {return buf;};
-    ~US() { uprv_free(buf);};
+       us.extract(0, us.length(), buf, bufLen); }
+    const char *cstr() {return buf;}
+    ~US() { uprv_free(buf);}
 };
 
 
diff --git a/icu4c/source/test/intltest/rbbimonkeytest.h b/icu4c/source/test/intltest/rbbimonkeytest.h
index 54d23fcceae..cff731ae6b6 100644
--- a/icu4c/source/test/intltest/rbbimonkeytest.h
+++ b/icu4c/source/test/intltest/rbbimonkeytest.h
@@ -148,8 +148,8 @@ class BreakRules: public UObject {
 
 class MonkeyTestData: public UObject {
   public:
-    MonkeyTestData() {};
-    ~MonkeyTestData() {};
+    MonkeyTestData() {}
+    ~MonkeyTestData() {}
     void set(BreakRules *rules, IntlTest::icu_rand &rand, UErrorCode &status);
     void clearActualBreaks();
     void dump(int32_t around = -1) const;
@@ -216,8 +216,8 @@ class RBBIMonkeyImpl: public UObject {
       private:
         RBBIMonkeyImpl *fMonkeyImpl;
       public:
-        RBBIMonkeyThread(RBBIMonkeyImpl *impl) : fMonkeyImpl(impl) {};
-        void run() U_OVERRIDE { fMonkeyImpl->runTest(); };
+        RBBIMonkeyThread(RBBIMonkeyImpl *impl) : fMonkeyImpl(impl) {}
+        void run() U_OVERRIDE { fMonkeyImpl->runTest(); }
     };
   private:
     void openBreakRules(const char *fileName, UErrorCode &status);
diff --git a/icu4c/source/test/intltest/regextst.cpp b/icu4c/source/test/intltest/regextst.cpp
index b1d191cdcc7..41afea4225c 100644
--- a/icu4c/source/test/intltest/regextst.cpp
+++ b/icu4c/source/test/intltest/regextst.cpp
@@ -4755,7 +4755,7 @@ struct callBackContext {
     int32_t          maxCalls;
     int32_t          numCalls;
     int32_t          lastSteps;
-    void reset(int32_t max) {maxCalls=max; numCalls=0; lastSteps=0;};
+    void reset(int32_t max) {maxCalls=max; numCalls=0; lastSteps=0;}
 };
 
 U_CDECL_BEGIN
@@ -4858,7 +4858,7 @@ struct progressCallBackContext {
     int64_t          lastIndex;
     int32_t          maxCalls;
     int32_t          numCalls;
-    void reset(int32_t max) {maxCalls=max; numCalls=0;lastIndex=0;};
+    void reset(int32_t max) {maxCalls=max; numCalls=0;lastIndex=0;}
 };
 
 // call-back function for find().
diff --git a/icu4c/source/test/intltest/simplethread.cpp b/icu4c/source/test/intltest/simplethread.cpp
index 4f833bbcbf1..f308958bed8 100755
--- a/icu4c/source/test/intltest/simplethread.cpp
+++ b/icu4c/source/test/intltest/simplethread.cpp
@@ -36,7 +36,7 @@ void SimpleThread::join() {
 
 class ThreadPoolThread: public SimpleThread {
   public:
-    ThreadPoolThread(ThreadPoolBase *pool, int32_t threadNum) : fPool(pool), fNum(threadNum) {};
+    ThreadPoolThread(ThreadPoolBase *pool, int32_t threadNum) : fPool(pool), fNum(threadNum) {}
     virtual void run() {fPool->callFn(fNum); }
     ThreadPoolBase *fPool;
     int32_t         fNum;
diff --git a/icu4c/source/test/intltest/simplethread.h b/icu4c/source/test/intltest/simplethread.h
index fae418dcc3b..22f245a5a92 100755
--- a/icu4c/source/test/intltest/simplethread.h
+++ b/icu4c/source/test/intltest/simplethread.h
@@ -76,8 +76,8 @@ class ThreadPool : public ThreadPoolBase {
     void (TestClass::*fRunFnPtr)(int32_t);
   public:
     ThreadPool(TestClass *test, int howMany, void (TestClass::*runFnPtr)(int32_t threadNumber)) :
-        ThreadPoolBase(test, howMany), fRunFnPtr(runFnPtr) {};
-    virtual ~ThreadPool() {};
+        ThreadPoolBase(test, howMany), fRunFnPtr(runFnPtr) {}
+    virtual ~ThreadPool() {}
   private:
     virtual void callFn(int32_t param) {
         TestClass *test = dynamic_cast<TestClass *>(fIntlTest);
diff --git a/icu4c/source/test/intltest/srchtest.cpp b/icu4c/source/test/intltest/srchtest.cpp
index a7534ddc64d..1eb728e4d22 100644
--- a/icu4c/source/test/intltest/srchtest.cpp
+++ b/icu4c/source/test/intltest/srchtest.cpp
@@ -2452,11 +2452,11 @@ void StringSearchTest::TestSubclass()
 class StubSearchIterator:public SearchIterator{
 public:
     StubSearchIterator(){}
-    virtual void setOffset(int32_t , UErrorCode &) {};
-    virtual int32_t getOffset(void) const {return 0;};
-    virtual SearchIterator* safeClone(void) const {return NULL;};
-    virtual int32_t handleNext(int32_t , UErrorCode &){return 0;};
-    virtual int32_t handlePrev(int32_t , UErrorCode &) {return 0;};
+    virtual void setOffset(int32_t , UErrorCode &) {}
+    virtual int32_t getOffset(void) const {return 0;}
+    virtual SearchIterator* safeClone(void) const {return NULL;}
+    virtual int32_t handleNext(int32_t , UErrorCode &){return 0;}
+    virtual int32_t handlePrev(int32_t , UErrorCode &) {return 0;}
     virtual UClassID getDynamicClassID() const {
         static char classID = 0;
         return (UClassID)&classID; 
diff --git a/icu4c/source/test/intltest/transapi.cpp b/icu4c/source/test/intltest/transapi.cpp
index 0e56a0fe333..48a85e5e1ad 100644
--- a/icu4c/source/test/intltest/transapi.cpp
+++ b/icu4c/source/test/intltest/transapi.cpp
@@ -970,7 +970,7 @@ class MyUnicodeFunctorTestClass : public UnicodeFunctor {
 public:
     virtual UnicodeFunctor* clone() const {return NULL;}
     static UClassID getStaticClassID(void) {return (UClassID)&MyUnicodeFunctorTestClassID;}
-    virtual UClassID getDynamicClassID(void) const {return getStaticClassID();};
+    virtual UClassID getDynamicClassID(void) const {return getStaticClassID();}
     virtual void setData(const TransliterationRuleData*) {}
 };
 
diff --git a/icu4c/source/test/intltest/transrt.cpp b/icu4c/source/test/intltest/transrt.cpp
index 44266425ec4..76542888e26 100644
--- a/icu4c/source/test/intltest/transrt.cpp
+++ b/icu4c/source/test/intltest/transrt.cpp
@@ -1282,7 +1282,7 @@ class LegalIndic :public Legal{
 public:
     LegalIndic();
     virtual UBool is(const UnicodeString& sourceString) const;
-    virtual ~LegalIndic() {};
+    virtual ~LegalIndic() {}
 };
 UBool LegalIndic::is(const UnicodeString& sourceString) const{
     int cp=sourceString.charAt(0);
diff --git a/icu4c/source/test/intltest/tsmthred.cpp b/icu4c/source/test/intltest/tsmthred.cpp
index 396c490f95d..0f4145b1d36 100644
--- a/icu4c/source/test/intltest/tsmthred.cpp
+++ b/icu4c/source/test/intltest/tsmthred.cpp
@@ -158,8 +158,8 @@ void MultithreadTest::TestThreads()
 class TestArabicShapeThreads : public SimpleThread
 {
 public:
-    TestArabicShapeThreads() {};
-    virtual void run() { doTailTest(); };
+    TestArabicShapeThreads() {}
+    virtual void run() { doTailTest(); }
 private:
 	void doTailTest();
 };
@@ -730,7 +730,7 @@ public:
         noLines(0),
         isAtLeastUCA62(TRUE)
     {
-    };
+    }
     void setCollator(Collator *c, Line *l, int32_t nl, UBool atLeastUCA62)
     {
         coll = c;
@@ -942,7 +942,7 @@ public:
         : SimpleThread(),
         fTraceInfo(0)
     {
-    };
+    }
 
 
     virtual void run()
@@ -1012,7 +1012,7 @@ void MultithreadTest::TestString()
 Transliterator *gSharedTranslit = NULL;
 class TxThread: public SimpleThread {
   public:
-    TxThread() {};
+    TxThread() {}
     ~TxThread();
     void run();
 };
@@ -1145,8 +1145,8 @@ class UnifiedCacheThread: public SimpleThread {
     UnifiedCacheThread(
             const UnifiedCache *cache,
             const char *loc,
-            const char *loc2) : fCache(cache), fLoc(loc), fLoc2(loc2) {};
-    ~UnifiedCacheThread() {};
+            const char *loc2) : fCache(cache), fLoc(loc), fLoc2(loc2) {}
+    ~UnifiedCacheThread() {}
     void run();
     void exerciseByLocale(const Locale &);
     const UnifiedCache *fCache;
@@ -1255,8 +1255,8 @@ static const UnicodeString *gTranslitExpected;
 
 class BreakTranslitThread: public SimpleThread {
   public:
-    BreakTranslitThread() {};
-    ~BreakTranslitThread() {};
+    BreakTranslitThread() {}
+    ~BreakTranslitThread() {}
     void run();
 };
 
@@ -1305,7 +1305,7 @@ void MultithreadTest::TestBreakTranslit() {
 
 class TestIncDecThread : public SimpleThread {
 public:
-    TestIncDecThread() { };
+    TestIncDecThread() {}
     virtual void run();
 };
 
@@ -1338,7 +1338,7 @@ static Calendar  *gSharedCalendar = {};
 
 class Test20104Thread : public SimpleThread {
 public:
-    Test20104Thread() { };
+    Test20104Thread() {}
     virtual void run();
 };
 
diff --git a/icu4c/source/test/intltest/tzfmttst.cpp b/icu4c/source/test/intltest/tzfmttst.cpp
index 056ab6cee47..8578019b546 100644
--- a/icu4c/source/test/intltest/tzfmttst.cpp
+++ b/icu4c/source/test/intltest/tzfmttst.cpp
@@ -410,7 +410,7 @@ struct LocaleData {
         for (int i=0; i<UPRV_LENGTHOF(times); i++) {
             times[i] = 0;
         }
-    };
+    }
 
     void resetTestIteration() {
         localeIndex = -1;
diff --git a/icu4c/source/test/iotest/filetst.c b/icu4c/source/test/iotest/filetst.c
index 1c667e174e1..3ac6674888f 100644
--- a/icu4c/source/test/iotest/filetst.c
+++ b/icu4c/source/test/iotest/filetst.c
@@ -1381,6 +1381,7 @@ static void TestFScanset(void) {
 #endif
 #if !UCONFIG_NO_FORMATTING
 static void TestBadFScanfFormat(const char *format, const UChar *uValue, const char *cValue) {
+    (void)cValue; // suppress compiler warnings about unused variable
     UFILE *myFile;
     UChar uBuffer[256];
     int32_t uNumScanned;
diff --git a/icu4c/source/test/iotest/strtst.c b/icu4c/source/test/iotest/strtst.c
index ce38aff1411..3962bde5d6f 100644
--- a/icu4c/source/test/iotest/strtst.c
+++ b/icu4c/source/test/iotest/strtst.c
@@ -687,6 +687,7 @@ static void TestSScanset(void) {
 
 static void TestBadSScanfFormat(const char *format, const UChar *uValue, const char *cValue) {
 #if !UCONFIG_NO_FORMATTING
+    (void)cValue; // suppress compiler warnings about unused variable
     UChar uBuffer[256];
     int32_t uNumScanned;
 
diff --git a/icu4c/source/tools/ctestfw/ctest.c b/icu4c/source/tools/ctestfw/ctest.c
index 5df32a91a05..f0f665428e4 100644
--- a/icu4c/source/tools/ctestfw/ctest.c
+++ b/icu4c/source/tools/ctestfw/ctest.c
@@ -891,18 +891,22 @@ log_data_err(const char* pattern, ...)
 static int traceFnNestingDepth = 0;
 U_CDECL_BEGIN
 static void U_CALLCONV TraceEntry(const void *context, int32_t fnNumber) {
+    (void)context; // suppress compiler warnings about unused variable
     char buf[500];
-    utrace_format(buf, sizeof(buf), traceFnNestingDepth*3, "%s() enter.\n", utrace_functionName(fnNumber));    buf[sizeof(buf)-1]=0;  
+    utrace_format(buf, sizeof(buf), traceFnNestingDepth*3, "%s() enter.\n", utrace_functionName(fnNumber));
+    buf[sizeof(buf)-1]=0;  
     fputs(buf, stdout);
     traceFnNestingDepth++;
 }   
  
-static void U_CALLCONV TraceExit(const void *context, int32_t fnNumber, const char *fmt, va_list args) {    char buf[500];
-    
+static void U_CALLCONV TraceExit(const void *context, int32_t fnNumber, const char *fmt, va_list args) {
+    (void)context; // suppress compiler warnings about unused variable
+    char buf[500];
     if (traceFnNestingDepth>0) {
         traceFnNestingDepth--; 
     }
-    utrace_format(buf, sizeof(buf), traceFnNestingDepth*3, "%s() ", utrace_functionName(fnNumber));    buf[sizeof(buf)-1]=0;
+    utrace_format(buf, sizeof(buf), traceFnNestingDepth*3, "%s() ", utrace_functionName(fnNumber));
+    buf[sizeof(buf)-1]=0;
     fputs(buf, stdout);
     utrace_vformat(buf, sizeof(buf), traceFnNestingDepth*3, fmt, args);
     buf[sizeof(buf)-1]=0;
@@ -912,6 +916,10 @@ static void U_CALLCONV TraceExit(const void *context, int32_t fnNumber, const ch
 
 static void U_CALLCONV TraceData(const void *context, int32_t fnNumber,
                           int32_t level, const char *fmt, va_list args) {
+    // suppress compiler warnings about unused variables
+    (void)context;  
+    (void)fnNumber;
+    (void)level;
     char buf[500];
     utrace_vformat(buf, sizeof(buf), traceFnNestingDepth*3, fmt, args);
     buf[sizeof(buf)-1]=0;
@@ -920,6 +928,7 @@ static void U_CALLCONV TraceData(const void *context, int32_t fnNumber,
 }
 
 static void *U_CALLCONV ctest_libMalloc(const void *context, size_t size) {
+    (void)context; // suppress compiler warnings about unused variable
     /*if (VERBOSITY) {
         printf("Allocated %ld\n", (long)size);
     }*/
@@ -929,6 +938,7 @@ static void *U_CALLCONV ctest_libMalloc(const void *context, size_t size) {
     return malloc(size);
 }
 static void *U_CALLCONV ctest_libRealloc(const void *context, void *mem, size_t size) {
+    (void)context; // suppress compiler warnings about unused variable
     /*if (VERBOSITY) {
         printf("Reallocated %ld\n", (long)size);
     }*/
@@ -939,6 +949,7 @@ static void *U_CALLCONV ctest_libRealloc(const void *context, void *mem, size_t
     return realloc(mem, size);
 }
 static void U_CALLCONV ctest_libFree(const void *context, void *mem) {
+    (void)context; // suppress compiler warnings about unused variable
     free(mem);
 }
 
diff --git a/icu4c/source/tools/gencnval/gencnval.c b/icu4c/source/tools/gencnval/gencnval.c
index dcc664b760d..82cf65d6a3a 100644
--- a/icu4c/source/tools/gencnval/gencnval.c
+++ b/icu4c/source/tools/gencnval/gencnval.c
@@ -582,6 +582,7 @@ addTaggedAlias(uint16_t tag, const char *alias, uint16_t converter) {
 
 static void
 addOfficialTaggedStandards(char *line, int32_t lineLen) {
+    (void) lineLen; // suppress compiler warnings about unused variable
     char *atag;
     char *endTagExp;
     char *tag;
diff --git a/icu4c/source/tools/gensprep/gensprep.c b/icu4c/source/tools/gensprep/gensprep.c
index a9c9efac95b..a78a5f3e566 100644
--- a/icu4c/source/tools/gensprep/gensprep.c
+++ b/icu4c/source/tools/gensprep/gensprep.c
@@ -278,6 +278,8 @@ static void U_CALLCONV
 normalizationCorrectionsLineFn(void *context,
                     char *fields[][2], int32_t fieldCount,
                     UErrorCode *pErrorCode) {
+    (void)context; // suppress compiler warnings about unused variable
+    (void)fieldCount; // suppress compiler warnings about unused variable
     uint32_t mapping[40];
     char *end, *s;
     uint32_t code;
@@ -341,6 +343,7 @@ static void U_CALLCONV
 strprepProfileLineFn(void *context,
               char *fields[][2], int32_t fieldCount,
               UErrorCode *pErrorCode) {
+    (void)fieldCount; // suppress compiler warnings about unused variable  
     uint32_t mapping[40];
     char *end, *map;
     uint32_t code;
diff --git a/icu4c/source/tools/gensprep/store.c b/icu4c/source/tools/gensprep/store.c
index 60252b9b903..3d75a39cc8c 100644
--- a/icu4c/source/tools/gensprep/store.c
+++ b/icu4c/source/tools/gensprep/store.c
@@ -464,7 +464,8 @@ storeMapping(uint32_t codepoint, uint32_t* mapping,int32_t length,
 
 
 extern void
-storeRange(uint32_t start, uint32_t end, UStringPrepType type,UErrorCode* status){
+storeRange(uint32_t start, uint32_t end, UStringPrepType type, UErrorCode* status){
+    (void)status; // suppress compiler warnings about unused variable
     uint16_t trieWord = 0;
 
     if((int)(_SPREP_TYPE_THRESHOLD + type) > 0xFFFF){
diff --git a/icu4c/source/tools/makeconv/gencnvex.c b/icu4c/source/tools/makeconv/gencnvex.c
index dcc849d8eda..726a1e5a86d 100644
--- a/icu4c/source/tools/makeconv/gencnvex.c
+++ b/icu4c/source/tools/makeconv/gencnvex.c
@@ -107,12 +107,17 @@ CnvExtClose(NewConverter *cnvData) {
 static UBool
 CnvExtIsValid(NewConverter *cnvData,
         const uint8_t *bytes, int32_t length) {
+    // suppress compiler warnings about unused variables
+    (void)cnvData;
+    (void)bytes;
+    (void)length;
     return FALSE;
 }
 
 static uint32_t
 CnvExtWrite(NewConverter *cnvData, const UConverterStaticData *staticData,
             UNewDataMemory *pData, int32_t tableType) {
+    (void) staticData; // suppress compiler warnings about unused variable
     CnvExtData *extData=(CnvExtData *)cnvData;
     int32_t length, top, headerSize;
 
@@ -121,7 +126,7 @@ CnvExtWrite(NewConverter *cnvData, const UConverterStaticData *staticData,
     if(tableType&TABLE_BASE) {
         headerSize=0;
     } else {
-        _MBCSHeader header={ { 0, 0, 0, 0 }, 0, 0, 0, 0, 0, 0, 0 };
+        _MBCSHeader header={ { 0, 0, 0, 0 }, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
 
         /* write the header and base table name for an extension-only table */
         length=(int32_t)uprv_strlen(extData->ucm->baseName)+1;
diff --git a/icu4c/source/tools/toolutil/ucbuf.h b/icu4c/source/tools/toolutil/ucbuf.h
index 81f8ba66333..116da37e9bc 100644
--- a/icu4c/source/tools/toolutil/ucbuf.h
+++ b/icu4c/source/tools/toolutil/ucbuf.h
@@ -32,11 +32,11 @@ typedef struct UCHARBUF UCHARBUF;
 /**
  * End of file value
  */
-#define U_EOF 0xFFFFFFFF
+#define U_EOF ((int32_t)0xFFFFFFFF)
 /**
  * Error value if a sequence cannot be unescaped
  */
-#define U_ERR 0xFFFFFFFE
+#define U_ERR ((int32_t)0xFFFFFFFE)
 
 typedef struct ULine ULine;