For investigation of the incidence of pulmonary thromboembolism, the sites of thromboemboli, pulmonary vascular changes, detailed clinical and autopsy records, and histological sections of a series of 103 autopsied adults were reviewed. Pulmonary thromboembolism was observed in 38.8% of these cases. Pulmonary thromboembolism tended to be more frequent in older, younger, female, and cancer patients. Thromboembolism was found most frequently in arterioles, followed by muscular arteries. A significant correlation (P less than 0.02) was found between the incidence of pulmonary thromboembolism and the histological changes of endothelial protrusion and vacuolization of the intima in these small pulmonary arteries, but no correlation was found between that of pulmonary thromboembolism and constriction figures. These endothelial changes probably represent previous vasoconstriction. The present observations suggest a significant relationship between pulmonary thromboembolism and vasoconstriction of small pulmonary arteries and suggest that widespread vasoconstriction of pulmonary arteries is important in the pathogenesis and pathophysiology of this condition.