diff --git a/app/trees/controller.py b/app/trees/controller.py
index 393c1fe..a0bc33a 100644
--- a/app/trees/controller.py
+++ b/app/trees/controller.py
@@ -174,6 +174,14 @@ def post(self, project_name: str):
             else:
                 message, passed = validate_ud(None, 3, data)
             return { "message": message if message != '---\n' else '', "passed": passed }
+        else: #for the moment if sud we check only the cycles
+            cycle_nodes = TreeService.check_cycle(data)
+            if cycle_nodes: 
+                error_message = 'Non tree structure, tokens: ' + ', '.join([str(tuple_nodes) for tuple_nodes in cycle_nodes]) + ' form a cycle'
+                return { "message": error_message, "passed": False }
+            else:
+                return { "message": '', "passed": True }               
+            
                       
 @api.route("/<string:project_name>/samples/<string:sample_name>/trees/all")
 class SaveAllTreesResource(Resource):
diff --git a/app/trees/service.py b/app/trees/service.py
index 38fecf7..cc4bd03 100644
--- a/app/trees/service.py
+++ b/app/trees/service.py
@@ -12,7 +12,28 @@
 VALIDATED = "validated"
 
 class TreeService:
-
+    
+    @staticmethod
+    def check_cycle(conll): 
+        sentence_json = sentenceConllToJson(conll)
+        nodes_json = sentence_json['treeJson']['nodesJson']
+        
+        nodes_children_list = {}
+        for index in nodes_json:
+            token_head = str(nodes_json[index]['HEAD'])
+            if token_head not in nodes_children_list.keys():
+                nodes_children_list[token_head] = [index]
+            else:
+                nodes_children_list[token_head].append(index)
+        
+        cycle_nodes = []        
+        for node, list_children in nodes_children_list.items():
+            for child in list_children:
+                if child in nodes_children_list.keys() and node in nodes_children_list[child]:
+                   cycle_nodes.append((child, node))
+        
+        return list(set(tuple(sorted(nodes_tuple)) for nodes_tuple in cycle_nodes))
+                       
     @staticmethod
     def samples_to_trees(samples, sample_name):
         """ transforms a list of samples into a trees object """