diff --git a/src/snmalloc/pal/pal_windows.h b/src/snmalloc/pal/pal_windows.h
index ebf2e68..776df89 100644
--- a/src/snmalloc/pal/pal_windows.h
+++ b/src/snmalloc/pal/pal_windows.h
@@ -9,7 +9,9 @@
 #    include <errno.h>
 #    include <stdio.h>
 #  endif
-#  define WIN32_LEAN_AND_MEAN
+#  ifndef WIN32_LEAN_AND_MEAN
+#    define WIN32_LEAN_AND_MEAN
+#  endif
 #  ifndef NOMINMAX
 #    define NOMINMAX
 #  endif