diff --git a/dlls/msvcr100/msvcr100.spec b/dlls/msvcr100/msvcr100.spec
index 82606e277886f3666e2f8ac3078c6318b35db1bd..64dc69a7f140f1974bbd2d95c9a39b391b772bc0 100644
--- a/dlls/msvcr100/msvcr100.spec
+++ b/dlls/msvcr100/msvcr100.spec
@@ -1197,7 +1197,7 @@
 @ cdecl _ui64toa(int64 ptr long) msvcrt._ui64toa
 @ cdecl _ui64toa_s(int64 ptr long long) msvcrt._ui64toa_s
 @ cdecl _ui64tow(int64 ptr long) msvcrt._ui64tow
-@ stub _ui64tow_s
+@ cdecl _ui64tow_s(int64 ptr long long) msvcrt._ui64tow_s
 @ cdecl _ultoa(long ptr long) msvcrt._ultoa
 @ cdecl _ultoa_s(long ptr long long) msvcrt._ultoa_s
 @ cdecl _ultow(long ptr long) msvcrt._ultow
diff --git a/dlls/msvcr80/msvcr80.spec b/dlls/msvcr80/msvcr80.spec
index e54eea6d6fa889d70ae139bfbff6f9c93e48ed91..e28079af5eb42225a989cd9025125f68bd389ae2 100644
--- a/dlls/msvcr80/msvcr80.spec
+++ b/dlls/msvcr80/msvcr80.spec
@@ -1050,7 +1050,7 @@
 @ cdecl _ui64toa(int64 ptr long) msvcrt._ui64toa
 @ cdecl _ui64toa_s(int64 ptr long long) msvcrt._ui64toa_s
 @ cdecl _ui64tow(int64 ptr long) msvcrt._ui64tow
-@ stub _ui64tow_s
+@ cdecl _ui64tow_s(int64 ptr long long) msvcrt._ui64tow_s
 @ cdecl _ultoa(long ptr long) msvcrt._ultoa
 @ cdecl _ultoa_s(long ptr long long) msvcrt._ultoa_s
 @ cdecl _ultow(long ptr long) msvcrt._ultow
diff --git a/dlls/msvcr90/msvcr90.spec b/dlls/msvcr90/msvcr90.spec
index 5baee987b1abf1d94805350e930060ab9783626a..94e32ff01fdac7db1b3b26099e7d061fbdc1cc14 100644
--- a/dlls/msvcr90/msvcr90.spec
+++ b/dlls/msvcr90/msvcr90.spec
@@ -1037,7 +1037,7 @@
 @ cdecl _ui64toa(int64 ptr long) msvcrt._ui64toa
 @ cdecl _ui64toa_s(int64 ptr long long) msvcrt._ui64toa_s
 @ cdecl _ui64tow(int64 ptr long) msvcrt._ui64tow
-@ stub _ui64tow_s
+@ cdecl _ui64tow_s(int64 ptr long long) msvcrt._ui64tow_s
 @ cdecl _ultoa(long ptr long) msvcrt._ultoa
 @ cdecl _ultoa_s(long ptr long long) msvcrt._ultoa_s
 @ cdecl _ultow(long ptr long) msvcrt._ultow
diff --git a/dlls/msvcrt/msvcrt.spec b/dlls/msvcrt/msvcrt.spec
index ba8c9ea171b298cacaf7cae4ead5c7213a3cd92d..a12941eea7debb6a1a5f80ede4aa0d81d1afe2ce 100644
--- a/dlls/msvcrt/msvcrt.spec
+++ b/dlls/msvcrt/msvcrt.spec
@@ -976,7 +976,7 @@
 @ cdecl _ui64toa(int64 ptr long) ntdll._ui64toa
 @ cdecl _ui64toa_s(int64 ptr long long) MSVCRT__ui64toa_s
 @ cdecl _ui64tow(int64 ptr long) ntdll._ui64tow
-# stub _ui64tow_s
+@ cdecl _ui64tow_s(int64 ptr long long) MSVCRT__ui64tow_s
 @ cdecl _ultoa(long ptr long) ntdll._ultoa
 @ cdecl _ultoa_s(long ptr long long)
 @ cdecl _ultow(long ptr long) ntdll._ultow
diff --git a/dlls/msvcrt/string.c b/dlls/msvcrt/string.c
index 11ad5a0c2d7e8f210c9c82949e34b1c5ccad21fe..6be223742f5cadd872ad90648488c7d86cd34767 100644
--- a/dlls/msvcrt/string.c
+++ b/dlls/msvcrt/string.c
@@ -918,6 +918,43 @@ int CDECL MSVCRT__ui64toa_s(unsigned __int64 value, char *str,
     return 0;
 }
 
+/*********************************************************************
+ *      _ui64tow_s  (MSVCRT.@)
+ */
+int CDECL MSVCRT__ui64tow_s( unsigned __int64 value, MSVCRT_wchar_t *str,
+                             MSVCRT_size_t size, int radix )
+{
+    MSVCRT_wchar_t buffer[65], *pos;
+    int digit;
+
+    if (!MSVCRT_CHECK_PMT(str != NULL) || !MSVCRT_CHECK_PMT(size > 0) ||
+        !MSVCRT_CHECK_PMT(radix>=2) || !MSVCRT_CHECK_PMT(radix<=36)) {
+        *MSVCRT__errno() = MSVCRT_EINVAL;
+        return MSVCRT_EINVAL;
+    }
+
+    pos = &buffer[64];
+    *pos = '\0';
+
+    do {
+	digit = value % radix;
+	value = value / radix;
+	if (digit < 10)
+	    *--pos = '0' + digit;
+	else
+	    *--pos = 'a' + digit - 10;
+    } while (value != 0);
+
+    if(buffer-pos+65 > size) {
+        MSVCRT_INVALID_PMT("str[size] is too small");
+        *MSVCRT__errno() = MSVCRT_EINVAL;
+        return MSVCRT_EINVAL;
+    }
+
+    memcpy(str, pos, buffer-pos+65);
+    return 0;
+}
+
 /*********************************************************************
  *  _ultoa_s (MSVCRT.@)
  */