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The lung phospholipid content and composition were determined on 20 nonbreathing human fetuses and abortuses ranging in gestational age from 2 to 9 months. The total phospholipids and phosphatidyl choline of the lung tissue remained relatively constant during the first 6 months of gestation, whereas the amount of disaturated phosphatidyl choline gradually increased during the same time. Between 6 and 9 months' gestation, the amounts of total phospholipid, phosphatidyl choline, and disaturated phosphatidyl choline all increased strikingly. The infants with hyaline membrane disease had lung tissue phospholipid values similar to those of fetuses of 6 months' gestation and “alveolar” surface concentrations of disaturated phosphatidyl choline far below that necessary to stabilize air spaces. The data are compatible with the hypothesis that hyaline membrane disease is due to a prenatal deficiency of lung surfactant.