In 11 pregnancies complicated by oligohydramnios due to spontaneous rupture of the membranes in the second trimester of pregnancy fetal breathing movements were assessed regularly by ultrasonographic examinations. In the 6 cases in which fetal breathing movements were detected the babies were liveborn and there was no evidence of pulmonary hypoplasia or the other non-renal features of Potter's syndrome. In the other 5 cases there were no fetal breathing movements. 1 pregnancy was terminated electively, and 1 ended in an intrauterine death; the remaining 3 infants died in the neonatal period. All 5 cases showed necropsy evidence of pulmonary hypoplasia. These findings indicate that premature and prolonged rupture of membranes in the second trimester of pregnancy does not uniformly result in a poor prognosis. They suggest that fetal breathing movements could be used as a predictor of favourable neonatal outcome.