Much progress has been made over the past 20 years in neonatal surgery, but the mortality rate in cases of congenital diaphragmatic hernia (CDH) for babies presenting in the early hours of life has remained high. We have reviewed our experience with special reference to 40 autopsied patients. Hypoplastic lungs were seen in all patients. When the ratio of observed combined lung weight to the expected combined lung weight is calculated, the result is 0.33 +/- 0.17 when the expected lung weights are calculated from the babies' birth weight; and 0.36 +/- 0.17 when the expected lung weights are calculated from the gestational age. In all patients with high ratios, extensive pneumonia was confirmed. Those patients with pneumonia were eliminated from the calculations. We cannot state unequivocably that hypoplasia of both lungs is the cause of death in all patients with CDH. We can affirm that, in our experience, hypoplasia is present and is probably a major factor in the high mortality rate.