In a series of 150 patients with congenital urologic disorders diagnosed in utero and managed in the neonatal period from 1980 to 1985, 48 cases were ureteropelvic junction obstructions. One was a false positive, and 47 were documented pyelocaliceal distension and retention; 12 of them were bilateral. Five cases had a spontaneous resolution. Three had a nephrectomy performed (destroyed kidney). Fifty kidneys have been operated on (Anderson-Hynes dysmembered pyeloplasty). Ninety percent of the cases are reported as clinically, biologically, and radiologically fair. Six percent postoperative complications are reported. The authors pointed out the great interest in neonatal repair of this condition, using microsurgical techniques without stent or nephrostomy. A comparison is made of the overall results with an identical series of older patients operated on during the same period.