diff --git a/src/os_macosx.m b/src/os_macosx.m
index 4ad8d26..1d88e7d 100644
--- a/src/os_macosx.m
+++ b/src/os_macosx.m
@@ -15,6 +15,10 @@
     Error: MACOS 9 is no longer supported in Vim 7
 #endif
 
+/* Avoid a conflict for the definition of Boolean between Mac header files and
+ * X11 header files. */
+#define NO_X11_INCLUDES
+
 #include "vim.h"
 #import <Cocoa/Cocoa.h>