Pulmonary tumor embolism to alveolar septal capillaries. A prospective study of 12 cases. Pulmonary tumor embolism leading to subacute cor pulmonale has been extensively studied. However, to our knowledge, the involvement of pulmonary alveolar capillaries has not previously been studied in detail. A prospective study was conducted on 112 autopsied patients with cancer to determine the presence of microembolism in the alveolar septal capillaries. Fifteen sections of predetermined lung areas were obtained and examined. Tumor embolism to alveolar septal capillaries was detected in 12 cases (10.7%), with simultaneous involvement of other vascular territories in 11. Seven patients had dyspnea; four, right ventricular failure; four, hypertrophy of the right ventricle; and three, arterial vascular sclerosis. The present data have led us to conclude that pulmonary tumor embolism to alveolar septal capillaries occurs in association with the involvement of arterial or lymphatic vessels and should be analyzed within the context of the study of subacute cor pulmonale. 