diff --git a/include/openssl/thread.h b/include/openssl/thread.h
index 1f8f524..f9b9980 100644
--- a/include/openssl/thread.h
+++ b/include/openssl/thread.h
@@ -64,6 +64,27 @@
 #endif
 
 
+#if defined(OPENSSL_WINDOWS)
+/* CRYPTO_MUTEX can appear in public header files so we really don't want to
+ * pull in windows.h. It's statically asserted that this structure is large
+ * enough to contain a Windows CRITICAL_SECTION by thread_win.c. */
+typedef union crypto_mutex_st {
+  double alignment;
+  uint8_t padding[4*sizeof(void*) + 2*sizeof(int)];
+} CRYPTO_MUTEX;
+#else
+/* It is reasonable to include pthread.h on non-Windows systems, however the
+ * |pthread_rwlock_t| that we need is hidden under feature flags, and we can't
+ * ensure that we'll be able to get it. It's statically asserted that this
+ * structure is large enough to contain a |pthread_rwlock_t| by
+ * thread_pthread.c. */
+typedef union crypto_mutex_st {
+  double alignment;
+  uint8_t padding[3*sizeof(int) + 5*sizeof(unsigned) + 16 + 8];
+} CRYPTO_MUTEX;
+#endif
+
+
 /* Functions to support multithreading.
  *
  * OpenSSL can safely be used in multi-threaded applications provided that at