diff --git a/flask/app.py b/flask/app.py
index b1ea0464..1d32ece9 100644
--- a/flask/app.py
+++ b/flask/app.py
@@ -839,6 +839,8 @@ class Flask(_PackageBoundObject):
         options.setdefault('use_reloader', self.debug)
         options.setdefault('use_debugger', self.debug)
         options.setdefault('passthrough_errors', True)
+        if debug:
+            self.jinja_env.auto_reload = True
         try:
             run_simple(host, port, self, **options)
         finally: