diff --git a/spec/StopwatchPluginSpec.php b/spec/StopwatchPluginSpec.php
index ba80ba6..755bcf8 100644
--- a/spec/StopwatchPluginSpec.php
+++ b/spec/StopwatchPluginSpec.php
@@ -7,10 +7,10 @@
 use Http\Promise\RejectedPromise;
 use Psr\Http\Message\RequestInterface;
 use Psr\Http\Message\ResponseInterface;
-use Psr\Http\Message\StreamInterface;
 use Psr\Http\Message\UriInterface;
 use Symfony\Component\Stopwatch\Stopwatch;
 use PhpSpec\ObjectBehavior;
+use Symfony\Component\Stopwatch\StopwatchEvent;
 
 class StopwatchPluginSpec extends ObjectBehavior
 {
@@ -29,14 +29,21 @@ function it_is_a_plugin()
         $this->shouldImplement('Http\Client\Common\Plugin');
     }
 
-    function it_records_event(Stopwatch $stopwatch, RequestInterface $request, ResponseInterface $response, UriInterface $uri)
-    {
+    function it_records_event(
+        Stopwatch $stopwatch,
+        RequestInterface $request,
+        ResponseInterface $response,
+        UriInterface $uri,
+        StopwatchEvent $event
+    ) {
         $request->getMethod()->willReturn('GET');
         $request->getUri()->willReturn($uri);
         $uri->__toString()->willReturn('http://foo.com/bar');
+        $event->getDuration()->willReturn(123);
 
         $stopwatch->start('GET http://foo.com/bar', 'php_http.request')->shouldBeCalled();
-        $stopwatch->stop('GET http://foo.com/bar', 'php_http.request')->shouldBeCalled();
+        $stopwatch->stop('GET http://foo.com/bar')->shouldBeCalled()->willReturn($event);
+        $response->withHeader('X-Duration', 123)->shouldBeCalled();
 
         $next = function (RequestInterface $request) use ($response) {
             return new FulfilledPromise($response->getWrappedObject());
@@ -52,7 +59,7 @@ function it_records_event_on_error(Stopwatch $stopwatch, RequestInterface $reque
         $uri->__toString()->willReturn('http://foo.com/bar');
 
         $stopwatch->start('GET http://foo.com/bar', 'php_http.request')->shouldBeCalled();
-        $stopwatch->stop('GET http://foo.com/bar', 'php_http.request')->shouldBeCalled();
+        $stopwatch->stop('GET http://foo.com/bar')->shouldBeCalled();
 
         $next = function (RequestInterface $request) {
             return new RejectedPromise(new NetworkException('', $request));
diff --git a/src/StopwatchPlugin.php b/src/StopwatchPlugin.php
index 8e3054a..f7e37c6 100644
--- a/src/StopwatchPlugin.php
+++ b/src/StopwatchPlugin.php
@@ -16,6 +16,7 @@
 final class StopwatchPlugin implements Plugin
 {
     const CATEGORY = 'php_http.request';
+    const HEADER = 'X-Duration';
 
     /**
      * @var Stopwatch
@@ -39,11 +40,11 @@ public function handleRequest(RequestInterface $request, callable $next, callabl
         $this->stopwatch->start($eventName, self::CATEGORY);
 
         return $next($request)->then(function (ResponseInterface $response) use ($eventName) {
-            $this->stopwatch->stop($eventName, self::CATEGORY);
+            $event = $this->stopwatch->stop($eventName);
 
-            return $response;
+            return $response->withHeader(self::HEADER, $event->getDuration());
         }, function (Exception $exception) use ($eventName) {
-            $this->stopwatch->stop($eventName, self::CATEGORY);
+            $this->stopwatch->stop($eventName);
 
             throw $exception;
         });