diff --git a/includes/form.inc b/includes/form.inc
index 8ca048a..167c5d6 100644
--- a/includes/form.inc
+++ b/includes/form.inc
@@ -525,8 +525,11 @@ function form_get_cache($form_build_id, &$form_state) {
  * Stores a form in the cache.
  */
 function form_set_cache($form_build_id, $form, $form_state) {
-  // 6 hours cache life time for forms should be plenty.
-  $expire = 21600;
+  // The default form cache lifetime is 6 hours. On busy sites, the cache_form
+  // table can become very large. A shorter form_cache lifetime means entries
+  // will expire more rapidly, which should help to keep the table's size under
+  // control.
+  $expire = variable_get('form_cache_expiry', 21600);
 
   // Cache form structure.
   if (isset($form)) {