diff --git a/openslides/agenda/static/js/agenda/site.js b/openslides/agenda/static/js/agenda/site.js
index a373d890d..f622bf870 100644
--- a/openslides/agenda/static/js/agenda/site.js
+++ b/openslides/agenda/static/js/agenda/site.js
@@ -585,6 +585,7 @@ angular.module('OpenSlidesApp.agenda.site', [
             $scope.items = AgendaTree.getTree(Agenda.getAll());
         });
         $scope.showInternalItems = true;
+        $scope.alert = {};
 
         // save parent and weight of moved agenda item (and all items on same level)
         $scope.treeOptions = {
@@ -594,7 +595,15 @@ angular.module('OpenSlidesApp.agenda.site', [
                 if (event.dest.nodesScope.item) {
                     parentID = event.dest.nodesScope.item.id;
                 }
-                $http.post('/rest/agenda/item/sort/', {nodes: event.dest.nodesScope.$modelValue, parent_id: parentID});
+                $http.post('/rest/agenda/item/sort/', {
+                    nodes: event.dest.nodesScope.$modelValue,
+                    parent_id: parentID}
+                ).then(
+                    function(success) {},
+                    function(error){
+                        $scope.alert = {type: 'danger', msg: error.data.detail, show: true};
+                    }
+                );
             }
         };
     }
diff --git a/openslides/agenda/static/templates/agenda/item-sort.html b/openslides/agenda/static/templates/agenda/item-sort.html
index eebe04d65..d92342624 100644
--- a/openslides/agenda/static/templates/agenda/item-sort.html
+++ b/openslides/agenda/static/templates/agenda/item-sort.html
@@ -19,6 +19,10 @@
       <translate ng-if="!showInternalItems">Show internal items</translate>
     </button>
 
+  <div uib-alert ng-show="alert.show" ng-class="'alert-' + (alert.type || 'warning')" ng-click="alert={}" close="alert={}">
+    {{ alert.msg }}
+  </div>
+
   <div ui-tree="treeOptions" id="tree-root">
     <ol ui-tree-nodes ng-model="items">
       <li ng-repeat="item in items" ui-tree-node ng-include="'nodes_renderer.html'">
diff --git a/openslides/agenda/views.py b/openslides/agenda/views.py
index 75d8d1cbe..b8a8e1c82 100644
--- a/openslides/agenda/views.py
+++ b/openslides/agenda/views.py
@@ -241,7 +241,8 @@ class ItemViewSet(ListModelMixin, RetrieveModelMixin, UpdateModelMixin, GenericV
     @list_route(methods=['post'])
     def sort(self, request):
         """
-        Sort agenda items.
+        Sort agenda items. Also checks parent field to prevent hierarchical
+        loops.
         """
         nodes = request.data.get('nodes', [])
         parent_id = request.data.get('parent_id')
@@ -253,5 +254,15 @@ class ItemViewSet(ListModelMixin, RetrieveModelMixin, UpdateModelMixin, GenericV
                 item.weight = index
                 item.save(skip_autoupdate=True)
                 items.append(item)
+
+                # Now check consistency. TODO: Try to use less DB queries.
+                item = Item.objects.get(pk=node['id'])
+                ancestor = item.parent
+                while ancestor is not None:
+                    if ancestor == item:
+                        raise ValidationError({'detail': _(
+                            'There must not be a hierarchical loop. Please reload the page.')})
+                    ancestor = ancestor.parent
+
         inform_changed_data(items)
         return Response({'detail': _('The agenda has been sorted.')})