Chronic massive thrombosis of major pulmonary arteries, usually described in 0.1% of autopsies, was found in 68 of 7,753 autopsies (0.9%) in our hospital. For comparison, 201 of the 7,753 autopsies (2.6%) revealed acute-massive pulmonary emboli. The 68 cases of chronic thromboses by definition had the right, left, or main pulmonary artery occluded or severely stenosed by organized thrombi. The average age was 67; males predominated 3 to 2. Dyspnea and right heart failure were the most common clinical features, though a correct antemortem diagnosis was rare. Severe lung disease was present in 34 cases (50%); one patient (1%) had mitral stenosis. In the remaining patients, clinical thrombophlebitis had been present in 20 cases (29%), and 13 cases (19%) had no recognized related disease. When the lung disease had prominent laterality, thrombi were invariably located on the side of the most serious disease, implying propagation of smaller thrombi in diseased lung retrograde to major arteries. In bilateral lung disease or in patients without lung disease, thromboses were randomly distributed between right or left sides, suggesting an embolic origin. Assessment of the severity of pulmonary arteriosclerosis and distal arterial patency indicated that most patients could have potentially benefited from surgical thromboendarterectomy. The reason for the high incidence of chronic pulmonary artery thrombosis in our autopsy service has not been elucidated. The effects of high altitude may be important in the pathogenesis, although polycythemia was not excessive in our cases.