--- a/OpenEXR/IlmImf/dwaLookups.cpp
+++ b/OpenEXR/IlmImf/dwaLookups.cpp
@@ -42,6 +42,9 @@
 #include <math.h>
 #include <vector>
 
+#ifdef _WIN32
+#include <windows.h>
+#endif
 #include <OpenEXRConfig.h>
 
 #ifdef OPENEXR_IMF_HAVE_SYSCONF_NPROCESSORS_ONLN
