Pulmonary tissues obtained from 12 individuals suffering from congenital heart disease associated with Down syndrome were examined by light and transmission electron microscopy and compared with those of 29 cases without the syndrome. Alkaline phosphatase (ALPase) activity, which is known to play an important role in the secretion of pulmonary surfactant, was histochemically examined and compared. The major changes found in the pulmonary tissues examined from pulmonary hypertensive patients were increase in the number of type II alveolar cells and in ALPase activity. ALPase activity was positive in the plasma membrane of type II cells and in the limiting membrane of the osmiophilic bodies contained within them. These changes were more conspicuous and detected at an earlier age in the pulmonary hypertensive individuals with Down syndrome than in those without the syndrome. These observations indicate that the acinar region in pulmonary tissue is affected at an earlier stage, and that changes in the production and secretion of pulmonary surfactant occur, in patients with Down syndrome. Early surgical treatment is recommended for them.