diff --git a/tommath.h b/tommath.h
index 7dda0a5..848431d 100644
--- a/tommath.h
+++ b/tommath.h
@@ -26,6 +26,12 @@
 extern "C" {
 #endif

+/* MS Visual C++ doesn't have a 128bit type for words, so fall back
+ * to 32bit MPI's (where words are 64bit) */
+#if defined(_MSC_VER) || defined(__LLP64__) || defined(__e2k__) || defined(__LCC__)
+#   define MP_32BIT
+#endif
+
 /* detect 64-bit mode if possible */
 #if defined(__x86_64__) || defined(_M_X64) || defined(_M_AMD64) || \
     defined(__powerpc64__) || defined(__ppc64__) || defined(__PPC64__) || \