Congenital diaphragmatic hernia, a highly lethal condition, displays at term the pulmonary biochemical and morphologic immaturity characteristic of premature delivery. We hypothesized that antenatal glucocorticoid, now the standard treatment to prevent hyaline membrane disease in premature human beings, might correct the parameters of the pulmonary biochemical and morphologic immaturity in severe congenital diaphragmatic hernia. A total of 112 fetal rats with or without nitrofcn-induced congenital diaphragmatic hernias from 34 pregnancies were treated antenatally with either saline or dexamethasone. Antenatal dexamethasone increased the lung disaturated phosphatidylcholine content, reduced the lung glycogen concentration, reduced the saccular septal thickness, and increased the mean saccular size and volume fraction of saccules in the lungs of rats with large congenital diaphragmatic hernia in comparison with similar rats not so treated. All differences were statistically significant. Antenatal glucocorticoid therapy was efficacious in treating rats with nitrofen-induced congenital diaphragmatic hernia. This encouraging finding warrants further investigation in a large animal model with surgically created congenital diaphragmatic hernia.