diff --git a/src/osd/windows/windows.mak b/src/osd/windows/windows.mak
index 21d6efeb9a3..10c7e8f743a 100644
--- a/src/osd/windows/windows.mak
+++ b/src/osd/windows/windows.mak
@@ -78,6 +78,18 @@ OBJDIRS += $(WINOBJ)
 
 
 
+#-------------------------------------------------
+# auto-detect PTR64
+#-------------------------------------------------
+
+ifndef PTR64
+ifneq (,$(findstring mingw64-w64,$(PATH)))
+PTR64=1
+endif
+endif
+
+
+
 #-------------------------------------------------
 # configure the resource compiler
 #-------------------------------------------------