diff --git a/dlls/msvcrt/mbcs.c b/dlls/msvcrt/mbcs.c
index 718e3871332..91efadd73dd 100644
--- a/dlls/msvcrt/mbcs.c
+++ b/dlls/msvcrt/mbcs.c
@@ -1058,18 +1058,22 @@ int CDECL _ismbbtrail(unsigned int c)
  */
 int CDECL _ismbslead(const unsigned char* start, const unsigned char* str)
 {
-  /* Lead bytes can also be trail bytes if caller messed up
-   * iterating through the string...
-   */
-  if(MSVCRT___mb_cur_max > 1)
-  {
-    while(start < str)
-      start += MSVCRT_isleadbyte(*str) ? 2 : 1;
+  int lead = 0;
 
-    if(start == str)
-      return MSVCRT_isleadbyte(*str);
+  if(!g_mbcp_is_multibyte)
+    return 0;
+
+  /* Lead bytes can also be trail bytes so we need to analise the string
+   */
+  while (start <= str)
+  {
+    if (!*start)
+      return 0;
+    lead = !lead && _ismbblead(*start);
+    start++;
   }
-  return 0; /* Must have been a trail, we skipped it */
+
+  return lead ? -1 : 0;
 }
 
 /*********************************************************************
@@ -1077,8 +1081,11 @@ int CDECL _ismbslead(const unsigned char* start, const unsigned char* str)
  */
 int CDECL _ismbstrail(const unsigned char* start, const unsigned char* str)
 {
-  /* Must not be a lead, and must be preceded by one */
-  return !_ismbslead(start, str) && MSVCRT_isleadbyte(str[-1]);
+  /* Note: this function doesn't check _ismbbtrail */
+  if ((str > start) && _ismbslead(start, str-1))
+    return -1;
+  else
+    return 0;
 }
 
 /*********************************************************************
diff --git a/dlls/msvcrt/tests/string.c b/dlls/msvcrt/tests/string.c
index 004d059c6f8..919964132d7 100644
--- a/dlls/msvcrt/tests/string.c
+++ b/dlls/msvcrt/tests/string.c
@@ -183,7 +183,7 @@ static void test_mbcp(void)
     int curr_mbcp = _getmbcp();
     unsigned char *mbstring = (unsigned char *)"\xb0\xb1\xb2 \xb3\xb4 \xb5"; /* incorrect string */
     unsigned char *mbstring2 = (unsigned char *)"\xb0\xb1\xb2\xb3Q\xb4\xb5"; /* correct string */
-    unsigned char *mbsonlylead = (unsigned char *)"\xb0\0\xb1\xb2";
+    unsigned char *mbsonlylead = (unsigned char *)"\xb0\0\xb1\xb2 \xb3";
     unsigned char buf[16];
     int step;
 
@@ -205,6 +205,40 @@ static void test_mbcp(void)
     ok(_ismbbtrail('\xb0'), "\xa0 should be a trail byte\n");
     ok(_ismbbtrail(' ') == FALSE, "' ' should not be a trail byte\n");
 
+    /* _ismbslead */
+    expect_eq(_ismbslead(mbstring, &mbstring[0]), -1, int, "%d");
+    expect_eq(_ismbslead(mbstring, &mbstring[1]), FALSE, int, "%d");
+    expect_eq(_ismbslead(mbstring, &mbstring[2]), -1, int, "%d");
+    expect_eq(_ismbslead(mbstring, &mbstring[3]), FALSE, int, "%d");
+    expect_eq(_ismbslead(mbstring, &mbstring[4]), -1, int, "%d");
+    expect_eq(_ismbslead(mbstring, &mbstring[5]), FALSE, int, "%d");
+    expect_eq(_ismbslead(mbstring, &mbstring[6]), FALSE, int, "%d");
+    expect_eq(_ismbslead(mbstring, &mbstring[7]), -1, int, "%d");
+    expect_eq(_ismbslead(mbstring, &mbstring[8]), FALSE, int, "%d");
+
+    expect_eq(_ismbslead(mbsonlylead, &mbsonlylead[0]), -1, int, "%d");
+    expect_eq(_ismbslead(mbsonlylead, &mbsonlylead[1]), FALSE, int, "%d");
+    expect_eq(_ismbslead(mbsonlylead, &mbsonlylead[2]), FALSE, int, "%d");
+    expect_eq(_ismbslead(mbsonlylead, &mbsonlylead[5]), FALSE, int, "%d");
+
+    /* _ismbstrail */
+    expect_eq(_ismbstrail(mbstring, &mbstring[0]), FALSE, int, "%d");
+    expect_eq(_ismbstrail(mbstring, &mbstring[1]), -1, int, "%d");
+    expect_eq(_ismbstrail(mbstring, &mbstring[2]), FALSE, int, "%d");
+    expect_eq(_ismbstrail(mbstring, &mbstring[3]), -1, int, "%d");
+    expect_eq(_ismbstrail(mbstring, &mbstring[4]), FALSE, int, "%d");
+    expect_eq(_ismbstrail(mbstring, &mbstring[5]), -1, int, "%d");
+    expect_eq(_ismbstrail(mbstring, &mbstring[6]), FALSE, int, "%d");
+    expect_eq(_ismbstrail(mbstring, &mbstring[7]), FALSE, int, "%d");
+    expect_eq(_ismbstrail(mbstring, &mbstring[8]), -1, int, "%d");
+
+    expect_eq(_ismbstrail(mbsonlylead, &mbsonlylead[0]), FALSE, int, "%d");
+    expect_eq(_ismbstrail(mbsonlylead, &mbsonlylead[1]), -1, int, "%d");
+    expect_eq(_ismbstrail(mbsonlylead, &mbsonlylead[2]), FALSE, int, "%d");
+    expect_eq(_ismbstrail(mbsonlylead, &mbsonlylead[3]), FALSE, int, "%d");
+    expect_eq(_ismbstrail(mbsonlylead, &mbsonlylead[4]), FALSE, int, "%d");
+    expect_eq(_ismbstrail(mbsonlylead, &mbsonlylead[5]), FALSE, int, "%d");
+
     /* _mbsnextc */
     expect_eq(_mbsnextc(mbstring), 0xb0b1, int, "%x");
     expect_eq(_mbsnextc(&mbstring[2]), 0xb220, int, "%x");  /* lead + invalid tail */