A male infant of 16 months, of the Han race, died from subacute infantile mountain sickness in Lhasa (3600 m). At necropsy there was right ventricular hypertrophy secondary to muscularization of the pulmonary arteries and arterioles thought to have been induced by hypobaric hypoxia. In addition, there was intimal proliferation of myofibroblasts in the pulmonary arterioles, venules and veins. There were increased numbers within the bronchioles of pulmonary endocrine cells, containing calcitonin and bombesin, which could be related to hypoxia or trophic effects on the pulmonary vasculature. The relation of delayed effects of hypoxia to primary pulmonary hypertension is considered in this study.