Obstetric sonography revealed urinary tract malformations in 13 fetuses. Six had severe dysplastic lesions incompatible with postnatal life; in all six, oligohydramnios and inability to detect normal kidney or bladder allowed appropriate counseling and management. Four fetuses had unilateral lesions (three hydronephrotic, one multicystic); all had evidence of adequate contralateral function and were successfully treated after delivery near term. Three fetuses had bilateral hydronephrosis secondary to urethral obstruction. The two who were born near term died of hypoplastic lungs, end-stage hydronephrosis, and facial and skeletal deformities. The other, electively delivered at 32 weeks, had none of the stigmata of Potter's syndrome, and early decompression salvaged sufficient renal function for survival. Prenatal sonographic assessment of urinary tract anatomy and function can improve perinatal management. The fetus with hydronephrosis may benefit from early decompression.