So called "congenital" hydronephrosis, a relatively frequent condition, is increasingly benefiting from modern investigative techniques such as ultrasound. This should enable a reduction in the time interval between clinical symptomatology and treatment. In a series of 93 cases, the authors demonstrate that pathological anatomy is of little value as regards etiology, while plastic surgery of the pyelo-ureteral junction still remains the most simple and reliable treatment with a low associated morbidity.