diff --git a/reactos/dll/win32/user32/misc/misc.c b/reactos/dll/win32/user32/misc/misc.c
index 7bd178afb6c..cce743eb29b 100644
--- a/reactos/dll/win32/user32/misc/misc.c
+++ b/reactos/dll/win32/user32/misc/misc.c
@@ -269,6 +269,29 @@ EndTask(
     return TRUE;
 }
 
+/*
+ * @implemented
+ */
+BOOL
+STDCALL
+IsGUIThread(
+    BOOL bConvert)
+{
+  PW32THREADINFO ti = (PW32THREADINFO)NtCurrentTeb()->Win32ThreadInfo;
+  if (ti == NULL)
+  {
+    if(bConvert)
+    {
+      if (NtUserGetThreadState(THREADSTATE_GETTHREADINFO)) return TRUE;
+      else
+         SetLastError(ERROR_NOT_ENOUGH_MEMORY);
+    }
+    return FALSE;
+  }
+  else
+    return TRUE;
+}
+
 PUSER_HANDLE_ENTRY
 FASTCALL
 GetUser32Handle(HANDLE handle)
diff --git a/reactos/dll/win32/user32/misc/stubs.c b/reactos/dll/win32/user32/misc/stubs.c
index e9ce40ac88d..ecad70006c5 100644
--- a/reactos/dll/win32/user32/misc/stubs.c
+++ b/reactos/dll/win32/user32/misc/stubs.c
@@ -539,18 +539,6 @@ GetRegisteredRawInputDevices(
   return FALSE;
 }
 
-/*
- * @unimplemented
- */
-BOOL
-STDCALL
-IsGUIThread(
-    BOOL bConvert)
-{
-  UNIMPLEMENTED;
-  return FALSE;
-}
-
 /*
  * @unimplemented
  */