Pulmonary hypoplasia was diagnosed in 14 of 88 infants (16%) delivered in 1983 to 1986 after rupture of the membranes of greater than or equal to 7 days with onset before 29 weeks' gestation. Logistic regression analysis applied to examine the relative importance of perinatal risk factors in the prediction of pulmonary hypoplasia showed that gestational age at onset of rupture of the membranes had a significant effect (p = 0.002) on the odds that pulmonary hypoplasia developed in the neonate, whereas the duration of rupture of the membranes (p = 0.11) and the degree of oligohydramnios (p = 0.65) did not. Postnatally, the presence of pulmonary hypoplasia was associated with the severity of skeletal compression deformities (p less than 0.0001). The development of skeletal compression deformities was associated with severe oligohydramnios (p = 0.05) and duration of rupture of the membranes (p = 0.02) but not gestational age at rupture of the membranes (p = 0.77). Gestational age at onset of rupture of the membranes being the best single predictor of pulmonary hypoplasia suggests that the stage of lung development at rupture of the membranes may be important.