diff --git a/bochs/iodev/pit82c54.h b/bochs/iodev/pit82c54.h
index 1b072ed22..f3d1cf0b6 100644
--- a/bochs/iodev/pit82c54.h
+++ b/bochs/iodev/pit82c54.h
@@ -8,6 +8,11 @@
 
 #include "bochs.h"
 
+#ifdef OUT
+// OUT is defined in system headers on cygwin, maybe others.
+#  undef OUT
+#endif
+
 class pit_82C54 : public logfunctions {
 
 public: