A series of 335 children with antenatally diagnosed hydronephrosis which persisted after the postnatal period was prospectively assessed. Idiopathic hydronephrosis (85 patients) and pelviureteric junction obstruction (73 patients) were the commonest conditions. All patients with idiopathic hydronephrosis were managed conservatively; 87% have had a complete resolution of hydronephrosis and 13% have remained unaltered. No patient managed conservatively has shown a deterioration in renal function.