diff --git mollom.module mollom.module
index e83fd7d..b281f0b 100644
--- mollom.module
+++ mollom.module
@@ -1696,6 +1696,11 @@ function mollom_pre_render_mollom($element) {
 
     // Prevent the page cache from storing a form containing a CAPTCHA element.
     $GLOBALS['conf']['cache'] = CACHE_DISABLED;
+    // Pressflow in CACHE_EXTERNAL caching mode additionally requires to mark
+    // this request as non-cacheable to bypass external caches (e.g., Varnish).
+    if (function_exists('drupal_page_is_cacheable')) {
+      drupal_set_header('Cache-Control', 'no-cache, must-revalidate, post-check=0, pre-check=0', FALSE);
+    }
 
     $data = array();
     if (!empty($form_state['mollom']['response']['session_id'])) {