diff --git a/index.js b/index.js
index 93f89f6..6fa83d5 100644
--- a/index.js
+++ b/index.js
@@ -55,7 +55,7 @@ module.exports = function (defaults) {
         cacheControl.push(util.format('s-maxage=%d', options.sMaxAge));
       }
 
-      if (cacheControl.length) {
+      if (cacheControl.length && !res.getHeader('Cache-Control')) {
         this.setHeader('Cache-Control', cacheControl.join(','));
       }
     });