Summary—In 22 of 40 patients with unilateral hydronephrosis the pressure flow study was compared with the outcome of surgery. Seventeen of the 22 patients had impaired function before surgery as judged from the 2‐min uptake on the renogram. The pressure flow study as a measure of pre‐operative obstruction showed no relation to the functional outcome of surgery. Patients with obstructed pressure flow studies as well as those without signs of obstruction either remained stable or improved after surgery.It was concluded that pressure flow studies are of no value in predicting the outcome of surgery for hydroneph