Pulmonary tumor embolism is a rare but well-documented cause of respiratory failure in patients with cancer. This entity is probably clinically underrecognized and may represent an important cause of mortality and morbidity in patients with cancer. Pulmonary tumor embolization may present at any stage of the patient's illness and indeed may be the first presentation of an occult carcinoma. In a review of 1069 nonmedicolegal autopsy protocols, we recently encountered three cases in which death had occurred from subacute cor pulmonale due to tumor embolization from breast, lung, and ovarian carcinoma. Recent advances in cytologic examination of blood samples obtained from Swan-Ganz catheters may prove useful in the diagnosis of this entity.