diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -775,6 +775,8 @@
   MetaVarName<"<arg>">;
 def Wall : Flag<["-"], "Wall">, Group<W_Group>, Flags<[CC1Option, HelpHidden]>;
 def WCL4 : Flag<["-"], "WCL4">, Group<W_Group>, Flags<[CC1Option, HelpHidden]>;
+def Wsystem_headers : Flag<["-"], "Wsystem-headers">, Group<W_Group>, Flags<[CC1Option, HelpHidden]>;
+def Wno_system_headers : Flag<["-"], "Wno-system-headers">, Group<W_Group>, Flags<[CC1Option, HelpHidden]>;
 def Wdeprecated : Flag<["-"], "Wdeprecated">, Group<W_Group>, Flags<[CC1Option]>,
   HelpText<"Enable warnings for deprecated constructs and define __DEPRECATED">;
 def Wno_deprecated : Flag<["-"], "Wno-deprecated">, Group<W_Group>, Flags<[CC1Option]>;
@@ -6475,6 +6477,11 @@
   Alias<W_Joined>, AliasArgs<["invalid-source-encoding"]>;
 def _SLASH_validate_charset_ : CLFlag<"validate-charset-">,
   Alias<W_Joined>, AliasArgs<["no-invalid-source-encoding"]>;
+def _SLASH_external_W0 : CLFlag<"external:W0">, HelpText<"Ignore warnings from system headers (default)">, Alias<Wno_system_headers>;
+def _SLASH_external_W1 : CLFlag<"external:W1">, HelpText<"Enable -Wsystem-headers">, Alias<Wsystem_headers>;
+def _SLASH_external_W2 : CLFlag<"external:W2">, HelpText<"Enable -Wsystem-headers">, Alias<Wsystem_headers>;
+def _SLASH_external_W3 : CLFlag<"external:W3">, HelpText<"Enable -Wsystem-headers">, Alias<Wsystem_headers>;
+def _SLASH_external_W4 : CLFlag<"external:W4">, HelpText<"Enable -Wsystem-headers">, Alias<Wsystem_headers>;
 def _SLASH_W0 : CLFlag<"W0">, HelpText<"Disable all warnings">, Alias<w>;
 def _SLASH_W1 : CLFlag<"W1">, HelpText<"Enable -Wall">, Alias<Wall>;
 def _SLASH_W2 : CLFlag<"W2">, HelpText<"Enable -Wall">, Alias<Wall>;
diff --git a/clang/test/Driver/cl-options.c b/clang/test/Driver/cl-options.c
--- a/clang/test/Driver/cl-options.c
+++ b/clang/test/Driver/cl-options.c
@@ -752,4 +752,12 @@
 // RUN: %clang_cl /JMC /Z7 /c -### -- %s 2>&1 | FileCheck %s --check-prefix JMC
 // JMC: -fjmc
 
+// RUN: %clang_cl /external:W0 /c -### -- %s 2>&1 | FileCheck -check-prefix=EXTERNAL_W0 %s
+// RUN: %clang_cl /external:W1 /c -### -- %s 2>&1 | FileCheck -check-prefix=EXTERNAL_Wn %s
+// RUN: %clang_cl /external:W2 /c -### -- %s 2>&1 | FileCheck -check-prefix=EXTERNAL_Wn %s
+// RUN: %clang_cl /external:W3 /c -### -- %s 2>&1 | FileCheck -check-prefix=EXTERNAL_Wn %s
+// RUN: %clang_cl /external:W4 /c -### -- %s 2>&1 | FileCheck -check-prefix=EXTERNAL_Wn %s
+// EXTERNAL_W0: "-Wno-system-headers"
+// EXTERNAL_Wn: "-Wsystem-headers"
+
 void f(void) { }