diff --git a/includes/module.inc b/includes/module.inc
index 0c06609..3748f60 100644
--- a/includes/module.inc
+++ b/includes/module.inc
@@ -747,18 +747,37 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
     // The hook is not cached, so ensure that whether or not it has
     // implementations, that the cache is updated at the end of the request.
     $implementations['#write_cache'] = TRUE;
+
     // Discover implementations for this hook.
-    $hook_info = module_hook_info();
     $implementations[$hook] = array();
-    $list = module_list(FALSE, FALSE, $sort);
-    foreach ($list as $module) {
-      $include_file = isset($hook_info[$hook]['group']) && module_load_include('inc', $module, $module . '.' . $hook_info[$hook]['group']);
-      // Since module_hook() may needlessly try to load the include file again,
-      // function_exists() is used directly here.
-      if (function_exists($module . '_' . $hook)) {
-        $implementations[$hook][$module] = $include_file ? $hook_info[$hook]['group'] : FALSE;
+    // Determine if a "group" is specified for this hook.
+    $hook_info = module_hook_info();
+    if (isset($hook_info[$hook]['group'])) {
+      // If a "group" is specified, implementations may live in lazy-loaded
+      // files named "$module.$group.inc".
+      $group = $hook_info[$hook]['group'];
+      foreach (module_list(FALSE, FALSE, $sort) as $module) {
+        // Include $module.$group.inc, if it exists.
+        $group_file_exists = module_load_include('inc', $module, "$module.$group");
+        // Determine whether the module implements the hook.
+        // Since module_hook() may needlessly try to load the include file
+        // again, function_exists() is used directly here.
+        if (function_exists($module . '_' . $hook)) {
+          $implementations[$hook][$module] = $group_file_exists ? $group : FALSE;
+        }
       }
     }
+    else {
+      // If no "group" is specified, module_implements() will not include any
+      // additional files.
+      foreach (module_list(FALSE, FALSE, $sort) as $module) {
+        // Determine whether the module implements the hook.
+        if (function_exists($module . '_' . $hook)) {
+          $implementations[$hook][$module] = FALSE;
+        }
+      }
+    }
+
     // Allow modules to change the weight of specific implementations, but avoid
     // an infinite loop.
     if ($hook != 'module_implements_alter') {