diff --git a/include/__mutex/once_flag.h b/include/__mutex/once_flag.h
index 086f75c..d66bb87 100644
--- a/include/__mutex/once_flag.h
+++ b/include/__mutex/once_flag.h
@@ -21,6 +21,10 @@
 #  include <tuple>
 #endif
 
+#if !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_CXX03_LANG)
+#include <atomic>
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
@@ -60,7 +64,11 @@ struct _LIBCPP_TEMPLATE_VIS once_flag {
   static const _State_type _Complete = ~_State_type(0);
 
 private:
+#if defined(_LIBCPP_ABI_MICROSOFT)
+  atomic<_State_type> __state_;
+#else
   _State_type __state_;
+#endif
 
 #ifndef _LIBCPP_CXX03_LANG
   template <class _Callable, class... _Args>
@@ -115,13 +123,21 @@ void _LIBCPP_HIDE_FROM_ABI __call_once_proxy(void* __vp) {
   (*__p)();
 }
 
+#ifdef _LIBCPP_ABI_MICROSOFT
+_LIBCPP_EXPORTED_FROM_ABI void __call_once(volatile atomic<once_flag::_State_type>&, void*, void (*)(void*));
+#else
 _LIBCPP_EXPORTED_FROM_ABI void __call_once(volatile once_flag::_State_type&, void*, void (*)(void*));
+#endif
 
 #ifndef _LIBCPP_CXX03_LANG
 
 template <class _Callable, class... _Args>
 inline _LIBCPP_HIDE_FROM_ABI void call_once(once_flag& __flag, _Callable&& __func, _Args&&... __args) {
+#if defined(_LIBCPP_ABI_MICROSOFT)
+  if (__flag.__state_.load(memory_order_acquire) != ~once_flag::_State_type(0)) {
+#else
   if (__libcpp_acquire_load(&__flag.__state_) != once_flag::_Complete) {
+#endif
     typedef tuple<_Callable&&, _Args&&...> _Gp;
     _Gp __f(_VSTD::forward<_Callable>(__func), _VSTD::forward<_Args>(__args)...);
     __call_once_param<_Gp> __p(__f);
@@ -133,7 +149,11 @@ inline _LIBCPP_HIDE_FROM_ABI void call_once(once_flag& __flag, _Callable&& __fun
 
 template <class _Callable>
 inline _LIBCPP_HIDE_FROM_ABI void call_once(once_flag& __flag, _Callable& __func) {
+#if defined(_LIBCPP_ABI_MICROSOFT)
+    if (__flag.__state_.load(memory_order_acquire) != ~once_flag::_State_type(0)) {
+#else
   if (__libcpp_acquire_load(&__flag.__state_) != once_flag::_Complete) {
+#endif
     __call_once_param<_Callable> __p(__func);
     std::__call_once(__flag.__state_, &__p, &__call_once_proxy<_Callable>);
   }
@@ -141,7 +161,11 @@ inline _LIBCPP_HIDE_FROM_ABI void call_once(once_flag& __flag, _Callable& __func
 
 template <class _Callable>
 inline _LIBCPP_HIDE_FROM_ABI void call_once(once_flag& __flag, const _Callable& __func) {
+#if defined(_LIBCPP_ABI_MICROSOFT)
+    if (__flag.__state_.load(memory_order_relaxed) != ~once_flag::_State_type(0)) {
+#else
   if (__libcpp_acquire_load(&__flag.__state_) != once_flag::_Complete) {
+#endif
     __call_once_param<const _Callable> __p(__func);
     std::__call_once(__flag.__state_, &__p, &__call_once_proxy<const _Callable>);
   }