We studied the lungs of seven patients of various ages who had Down's syndrome, to determine whether they had abnormalities in pulmonary development. Six of the seven had hypoplastic lungs. Five had congenital heart disease, but pulmonary hypoplasia was of equal severity, irrespective of the presence or absence or the type of congenital heart disease. Three other patients with congenital heart disease but without Down's syndrome had lungs that were equally diminished in volume. However, these lungs lacked the structural abnormalities seen in Down's syndrome, which consisted of a diminished number of alveoli in relation to acini and enlarged alveoli and alveolar ducts. The patients with Down's syndrome also had a smaller total number of alveoli and a smaller alveolar surface area. We speculate that the smaller alveolar surface area is accompanied by loss of capillary surface area, which is responsible for the aggravation of pulmonary hypertension in Down's syndrome.