This paper reports the histopathologic and ultrastructural changes in 37 children and 20 adults with high altitude heart disease. Its clinicopathologic features, diagnostic criteria and differential diagnosis from Ke-shan disease and other heart diseases are discussed. The right heart of 37 children showed hypertrophy and dilatation. Microscopically, necrosis and scarring were found mainly in the right ventricular wall (29/37) and the anterior papillary muscles (29/37) of the right ventricle. The hearts of 20 adults (19 Hans and 1 Tibetan) were enlarged and increased in weight. Heart weight increased to 400-500g in 15 cases, and hypertrophy of both ventricles was seen in 16 cases. Necrosis and scarring were found mainly in the papillary muscles (20:16) and the ventricular walls (11:19) of the left and right ventricles. Electronmicroscopy showed that myofibrils were dissolved or degenerated, mitochondria swelled, endoplasmic reticula dilated and glycogen granules decreased. As regards the pathogenesis, the authors stress the role of chronic hypoxia which causes myocardial damage, and advocate the importance of early diagnosis and treatment.