The analysis of urine obtained from fetuses with hydronephrosis, seen on ultrasound, can give a misleading assessment of residual renal function. Additional parameters for assessment of fetal renal function would be helpful. We have used SDS-polyacrylamide gel electrophoresis to separate urinary proteins from a fetus with obstructive uropathy and severe oligohydramnios, already present at 18 weeks of gestation. The dilated urinary bladder of the fetus was successfully shunted in utero with a double pigtail catheter which worked for 17 weeks, and a boy without renal or pulmonary insufficiency was born at 36 weeks. In this case the prenatal protein analysis by electrophoresis was a better indicator of the ultimate good pregnancy outcome than the evaluation of urinary electrolytes and osmolarity alone. We therefore suggest the addition of this test to the profile of renal function studies performed on fetal urine.