Index: includes/api/ApiQueryCategories.php
===================================================================
--- includes/api/ApiQueryCategories.php	(revision 44583)
+++ includes/api/ApiQueryCategories.php	(working copy)
@@ -96,9 +96,15 @@
 			$this->dieUsage("Incorrect parameter - mutually exclusive values may not be supplied", 'show');
 		if(isset($show['hidden']) || isset($show['!hidden']))
 		{
-			$this->addTables('category');
-			$this->addWhere(array(	'cl_to = cat_title',
-						'cat_hidden' => isset($show['hidden'])));
+			$this->addOption('STRAIGHT_JOIN');
+			$this->addTables(array('page', 'page_props'));
+			$this->addJoinConds(array(
+				'page' => array('LEFT JOIN', 'page_namespace=14 AND page_title=cl_to'),
+				'page_props' => array('LEFT JOIN', 'pp_page=page_id AND pp_propname=\'hiddencat\'')));
+			if(isset($show['hidden']))
+				$this->addWhere(array('pp_propname IS NOT NULL'));
+			else
+				$this->addWhere(array('pp_propname IS NULL'));
 		}
 
 		# Don't order by cl_from if it's constant in the WHERE clause