diff --git a/reactos/lib/sdk/crt/stdlib/mbstowcs.c b/reactos/lib/sdk/crt/stdlib/mbstowcs.c
index ba7e4c12bef..1fe9917f332 100644
--- a/reactos/lib/sdk/crt/stdlib/mbstowcs.c
+++ b/reactos/lib/sdk/crt/stdlib/mbstowcs.c
@@ -1,28 +1,43 @@
 #include <precomp.h>
 
+/*********************************************************************
+ * _mbstowcs_l
+ */
+size_t CDECL _mbstowcs_l(wchar_t *wcstr, const char *mbstr,
+        size_t count, _locale_t locale)
+{
+    MSVCRT_pthreadlocinfo locinfo;
+    size_t i, size;
+
+    if(!locale)
+        locinfo = get_locinfo();
+    else
+        locinfo = ((MSVCRT__locale_t)locale)->locinfo;
+
+    /* Ignore count parameter */
+    if(!wcstr)
+        return MultiByteToWideChar(locinfo->lc_codepage, 0, mbstr, -1, NULL, 0)-1;
+
+    for(i=0, size=0; i<count; i++) {
+        if(mbstr[size] == '\0')
+            break;
+
+        size += (_isleadbyte_l((unsigned char)mbstr[size], locale) ? 2 : 1);
+    }
+
+    size = MultiByteToWideChar(locinfo->lc_codepage, 0,
+            mbstr, size, wcstr, count);
+
+    if(size<count && wcstr)
+        wcstr[size] = '\0';
+
+    return size;
+}
 
 /*
  * @implemented
  */
 size_t mbstowcs (wchar_t *widechar, const char *multibyte, size_t number)
 {
-    int bytes;
-    size_t n = 0;
-
-    while (n < number) {
-
-	if ((bytes = mbtowc (widechar, multibyte, MB_LEN_MAX)) < 0)
-	    return (size_t) -1;
-
-	if (bytes == 0) {
-	    *widechar = (wchar_t) '\0';
-	    return n;
-	}
-
-	widechar++;
-	multibyte += bytes;
-	n++;
-    }
-
-    return n;
+    return _mbstowcs_l(widechar, multibyte, number, NULL);
 }