diff --git a/win32/Makefile.msvc b/win32/Makefile.msvc
index 11b76f4..013e6d0 100644
--- a/win32/Makefile.msvc
+++ b/win32/Makefile.msvc
@@ -68,6 +68,9 @@
 # The preprocessor and its options.
 CPP = cl.exe /EP
 CPPFLAGS = /nologo /I$(XML_SRCDIR)\include
+!if "$(WITH_THREADS)" == "1"
+CPPFLAGS = $(CPPFLAGS) /D "_REENTRANT"
+!endif
 
 # The compiler and its options.
 CC = cl.exe