diff --git a/src/msw/utilsexc.cpp b/src/msw/utilsexc.cpp
index e9f7ae28b5..7198e1106f 100644
--- a/src/msw/utilsexc.cpp
+++ b/src/msw/utilsexc.cpp
@@ -82,6 +82,15 @@
 
 #include "wx/msw/private/hiddenwin.h"
 
+// FIXME-VC6: These are not defined in VC6 SDK headers.
+#ifndef BELOW_NORMAL_PRIORITY_CLASS
+    #define BELOW_NORMAL_PRIORITY_CLASS 0x4000
+#endif
+
+#ifndef ABOVE_NORMAL_PRIORITY_CLASS
+    #define ABOVE_NORMAL_PRIORITY_CLASS 0x8000
+#endif
+
 // ----------------------------------------------------------------------------
 // constants
 // ----------------------------------------------------------------------------