Pulmonary embolism discovered at autopsy is still as prevalent as previously reported in the last three to four decades. Only a certain percentage of pulmonary emboli result in pulmonary infarction. Recently published studies have suggested that importance of the size of the occluded pulmonary artery in the occurrence of infarction. Our study of 45 autopsy subjects in which there were pulmonary emboli shows a 31 percent incidence of pulmonary artery branches of 3 mm in diameter or less, but emboli in larger arteries may show frequent extensions into their smaller distal branches without producing infarct. Pulmonary infarction also occurs more commonly in patients dying of cardiovascular or malignant diseases than it does in those dying of other diseases, and the combination of shock and congestive left heart failure appears to be the most significant hemodynamic risk factor in the development of pulmonary infarction. However, the increased risk of pulmonary infarction in patients with malignancy may not be accounted for by the existence of these two hemodynamic risk factors alone.