Hypoplasia of the fetal lung is found in association with long-standing oligohydramnios. In animal models, interference with fetal breathing activity has led to lung hypoplasia. It has been suggested that lung hypoplasia associated with oligohydramnios is due to inhibition of fetal breathing. Observations of seven patients with prolonged oligohydramnios, three of whom had lung hypoplasia, indicate that fetal breathing does occur in these cases and that is unlikely that lung hypoplasia is merely the result of absent fetal breathing activity. In fact, when compared to the four patients with oligohydramnios and presumably normal lungs, the three patients with oligohydramnios and lung hypoplasia spent more time breathing and did so at higher rates.