Nine patients with a prenatal ultrasonic diagnosis of unilateral ureteropelvic junction obstruction underwent pyeloplasty in early neonatal life. Based on radiological and renographical assessment of the results, immediate postnatal intervention had no demonstrable advantage over those subjects in a control group who underwent an operation after presenting with symptoms. Half of the children in both groups had normal parenchymal function of the hydronephrotic kidney. Because of the operative complications we found it unacceptable to operate on neonates without symptoms and with normal function of the affected kidney. The advantage of prenatal diagnosis lies in the early recognition of the necessity of close surveillance. The primary indications for surgical intervention should be symptoms and impairment of the hydronephrotic kidney function.