diff --git a/icu4c/source/i18n/tolowtrn.cpp b/icu4c/source/i18n/tolowtrn.cpp
index a0e1380e79..954d880f13 100644
--- a/icu4c/source/i18n/tolowtrn.cpp
+++ b/icu4c/source/i18n/tolowtrn.cpp
@@ -75,32 +75,35 @@ void LowercaseTransliterator::handleTransliterate(Replaceable& text,
     UnicodeString original;
     text.extractBetween(offsets.contextStart, offsets.contextLimit, original);
     
+    UCharIterator iter;
+    uiter_setReplaceable(&iter, &text);
+    iter.start = offsets.contextStart;
+    iter.limit = offsets.contextLimit;
+
     // Walk through original string
     // If there is a case change, modify corresponding position in replaceable
     
     int32_t i = textPos - offsets.contextStart;
     int32_t limit = offsets.limit - offsets.contextStart;
-    UChar32 cp, bufferCH;
+    UChar32 cp;
     int32_t oldLen;
     
     for (; i < limit; ) { 
         UErrorCode status = U_ZERO_ERROR;
         int32_t s = i;
-        bufferCH = original.char32At(s);
 
         UTF_GET_CHAR(original.getBuffer(), 0, i, original.length(), cp);
         oldLen = UTF_CHAR_LENGTH(cp);
         i += oldLen;
-        int32_t len = u_strToLower(buffer, u_getMaxCaseExpansion(), original.getBuffer()+s, i-s, loc.getName(), &status);
-        /* Skip checking of status code here because the buffer should not have overflowed. */
-        UTF_GET_CHAR(buffer, 0, 0, len, cp);
-        if ( bufferCH != cp ) {
-            UnicodeString temp(buffer);
+        iter.index = i; // Point _past_ current char
+        int32_t newLen = u_internalToLower(cp, &iter, buffer, u_getMaxCaseExpansion(), loc.getName());
+        if (newLen >= 0) {
+            UnicodeString temp(buffer, newLen);
             text.handleReplaceBetween(textPos, textPos + oldLen, temp);
-            if (len != oldLen) {
-                textPos += len;
-                offsets.limit += len - oldLen;
-                offsets.contextLimit += len - oldLen;
+            if (newLen != oldLen) {
+                textPos += newLen;
+                offsets.limit += newLen - oldLen;
+                offsets.contextLimit += newLen - oldLen;
                 continue;
             }
         }
diff --git a/icu4c/source/i18n/toupptrn.cpp b/icu4c/source/i18n/toupptrn.cpp
index 2f215c2704..b0ae0cc896 100644
--- a/icu4c/source/i18n/toupptrn.cpp
+++ b/icu4c/source/i18n/toupptrn.cpp
@@ -77,33 +77,36 @@ void UppercaseTransliterator::handleTransliterate(Replaceable& text,
     
     UnicodeString original;
     text.extractBetween(offsets.contextStart, offsets.contextLimit, original);
+    
+    UCharIterator iter;
+    uiter_setReplaceable(&iter, &text);
+    iter.start = offsets.contextStart;
+    iter.limit = offsets.contextLimit;
             
     // Walk through original string
     // If there is a case change, modify corresponding position in replaceable
     
     int32_t i = textPos - offsets.contextStart;
     int32_t limit = offsets.limit - offsets.contextStart;
-    UChar32 cp, bufferCH;
+    UChar32 cp;
     int32_t oldLen;
     
     for (; i < limit; ) {
         UErrorCode status = U_ZERO_ERROR;
         int32_t s = i;
-        bufferCH = original.char32At(s);
 
         UTF_GET_CHAR(original.getBuffer(), 0, i, original.length(), cp);
         oldLen = UTF_CHAR_LENGTH(cp);
         i += oldLen;
-        int32_t len = u_strToUpper(buffer, u_getMaxCaseExpansion(), original.getBuffer()+s, i-s, loc.getName(), &status);
-        /* Skip checking of status code here because the buffer should not have overflowed. */
-        UTF_GET_CHAR(buffer, 0, 0, len, cp);
-        if (bufferCH != cp) {
-            UnicodeString temp(buffer);
+        iter.index = i; // Point _past_ current char
+        int32_t newLen = u_internalToUpper(cp, &iter, buffer, u_getMaxCaseExpansion(), loc.getName());
+        if (newLen >= 0) {
+            UnicodeString temp(buffer, newLen);
             text.handleReplaceBetween(textPos, textPos + oldLen, temp);
-            if (len != oldLen) {
-                textPos += len;
-                offsets.limit += len - oldLen;
-                offsets.contextLimit += len - oldLen;
+            if (newLen != oldLen) {
+                textPos += newLen;
+                offsets.limit += newLen - oldLen;
+                offsets.contextLimit += newLen - oldLen;
                 continue;
             }
         }