diff --git a/tools/c-index-test/c-index-test.c b/tools/c-index-test/c-index-test.c
index efff1ae..7300585 100644
--- a/tools/c-index-test/c-index-test.c
+++ b/tools/c-index-test/c-index-test.c
@@ -179,12 +179,17 @@
 }
 
 int perform_test_load_source(int argc, const char **argv, const char *filter) {
+  const char *UseExternalASTs =
+    getenv("CINDEXTEST_USE_EXTERNAL_AST_GENERATION");
   CXIndex Idx;
   CXTranslationUnit TU;
   Idx = clang_createIndex(/* excludeDeclsFromPCH */
                           !strcmp(filter, "local") ? 1 : 0,
                           /* displayDiagnostics */ 1);
 
+  if (UseExternalASTs && strlen(UseExternalASTs))
+    clang_setUseExternalASTGeneration(Idx, 1);
+
   TU = clang_createTranslationUnitFromSourceFile(Idx, 0, argc, argv);
   if (!TU) {
     fprintf(stderr, "Unable to load translation unit!\n");