diff --git a/lib/base/heimbase.c b/lib/base/heimbase.c
index 4848bd5b7..8f5694d27 100644
--- a/lib/base/heimbase.c
+++ b/lib/base/heimbase.c
@@ -368,7 +368,16 @@ _heim_type_get_tid(heim_type_t type)
 void
 heim_base_once_f(heim_base_once_t *once, void *ctx, void (*func)(void *))
 {
-#ifdef HAVE_DISPATCH_DISPATCH_H
+#if defined(WIN32)
+    if (InterlockedCompareExchange(&once->started, 1L, 0L) == 0L) {
+	once->running = 1L;
+	(*func)(ctx);
+	once->running = 0L;
+    } else {
+	while (once->running)
+	    SwitchToThread();
+    }
+#elif defined(HAVE_DISPATCH_DISPATCH_H)
     dispatch_once_f(once, ctx, func);
 #else
     static HEIMDAL_MUTEX mutex = HEIMDAL_MUTEX_INITIALIZER;
diff --git a/lib/base/heimbase.h b/lib/base/heimbase.h
index 6d4332509..c617a6344 100644
--- a/lib/base/heimbase.h
+++ b/lib/base/heimbase.h
@@ -56,8 +56,16 @@ typedef void * heim_object_t;
 typedef unsigned int heim_tid_t;
 typedef heim_object_t heim_bool_t;
 typedef heim_object_t heim_null_t;
-#define HEIM_BASE_ONCE_INIT 0
+#if defined(WIN32)
+typedef struct {
+    LONG started;
+    LONG running;
+} heim_base_once_t;
+# define HEIM_BASE_ONCE_INIT {0L, 0L}
+#else
+# define HEIM_BASE_ONCE_INIT 0
 typedef long heim_base_once_t; /* XXX arch dependant */
+#endif
 
 #if !defined(__has_extension)
 #define __has_extension(x) 0