diff --git a/icu4c/source/common/unicode/udata.h b/icu4c/source/common/unicode/udata.h
index b864010f6f1..0881ef32453 100644
--- a/icu4c/source/common/unicode/udata.h
+++ b/icu4c/source/common/unicode/udata.h
@@ -371,7 +371,8 @@ typedef enum UDataFileAccess {
 /**
  * This function may be called to control how ICU loads data. It must be called
  * before any ICU data is loaded, including application data loaded with ures/ResourceBundle or
- * udata APIs. It should be called before u_init.
+ * udata APIs. It should be called before u_init.  This function is not multithread safe.  
+ * The results of calling it while other threads are loading data are undefined.
  * @param access The type of file access to be used
  * @param status Error code.
  * @see UDataFileAccess