(1) Out of 415 autopsied cases in Yokufukai Hospital (old people's home) during last 4 years, 28 cases were given the pathological diagnosis of pulmonary heart diseases: 23 cases of pulmonary emphysema, 3 cases of pulmonary tuberculosis, 1 case of pulmonary fibrosis and 1 case of pulmonary embolism. Although the incidence of pulmonary emphysema is much greater in men than in women, no marked sex difference was found in the incidence of pulmonary heart disease among patients with pulmonary emphysema, pulmonary heart disease occurring in about 20% regardless of the sex. In pulmonary tuberculosis, pulmonary heart disease is seen only in far advanced cases with long standing history.(2) Pulmonary atherosclerosis in the major arteries and thickening of the intima and media throughout the arterioles, muscular arteries and elastic arteries of the pulmonary vessels were very common in pulmonary heart disease. Histological examination of the pulmonary arterial wall in these cases revealed intimal fibrosis and increase of elastica in the media in the muscular pulmonary arteries. These changes probably represent secondary manifestations of the disturbances in pulmonary circulation.(3) The aetiologic factors in the production of right heart hypertrophy in several lung diseases have been studied by standard histological methods and by post-mortem pulmonary arteriography. In the cases of pulmonary tumor emboli, primary pulmonary hypertension, pulmonary scleroderma and pulmonary tuberculosis, a marked reduction was observed in the vascular cross section due to obliteration or narrowing in a sufficient extent to raise the pulmonary arterial pressure.(4) Marked decrease in the number of the small branches of pulmonary arteries was found in the arteriograms in every type of pulmonary emphysema, although the mode of destruction of the arteries is different between the centrilobular and panacinar types of emphysema; in the centrilobular emphysema the central part of the lobule is destroyed while in the panacinar emphysema the destruction is mainly seen in the peripheral part of the lobule. No marked difference, however, was found in the rate of the development of pulmonary heart disease between these 2 types of emphysema. The most important organic changes to reduce the large reserve of the vascular bed of the lung in emphysema appears to be the loss of the pulmonary vascular bed.