Index: includes/api/ApiQueryAllCategories.php
===================================================================
--- includes/api/ApiQueryAllCategories.php	(revision 44583)
+++ includes/api/ApiQueryAllCategories.php	(working copy)
@@ -67,7 +67,19 @@
 
 		$prop = array_flip($params['prop']);
 		$this->addFieldsIf( array( 'cat_pages', 'cat_subcats', 'cat_files' ), isset($prop['size']) );
-		$this->addFieldsIf( 'cat_hidden', isset($prop['hidden']) );
+		if(isset($prop['hidden']))
+		{
+			$this->addTables(array('page', 'page_props'));
+			$this->addJoinConds(array(
+				'page' => array('LEFT JOIN', array(
+					'page_namespace' => NS_CATEGORY,
+					'page_title=cat_title')),
+				'page_props' => array('LEFT JOIN', array(
+					'pp_page=page_id',
+					'pp_propname' => 'hiddencat')),
+			));
+			$this->addFields('pp_propname AS cat_hidden');
+		}
 
 		$res = $this->select(__METHOD__);
 
Index: includes/api/ApiQueryCategoryInfo.php
===================================================================
--- includes/api/ApiQueryCategoryInfo.php	(revision 44583)
+++ includes/api/ApiQueryCategoryInfo.php	(working copy)
@@ -55,8 +55,16 @@
 			$cattitles[$c] = $t->getDBKey();
 		}
 
-		$this->addTables('category');
-		$this->addFields(array('cat_title', 'cat_pages', 'cat_subcats', 'cat_files', 'cat_hidden'));
+		$this->addTables(array('category', 'page', 'page_props'));
+		$this->addJoinConds(array(
+			'page' => array('LEFT JOIN', array(
+				'page_namespace' => NS_CATEGORY,
+				'page_title=cat_title')),
+			'page_props' => array('LEFT JOIN', array(
+				'pp_page=page_id',
+				'pp_propname' => 'hiddencat')),
+		));
+		$this->addFields(array('cat_title', 'cat_pages', 'cat_subcats', 'cat_files', 'pp_propname AS cat_hidden'));
 		$this->addWhere(array('cat_title' => $cattitles));			
 
 		$db = $this->getDB();