Tumor embolization was found at autopsy in the pulmonary arteries of 33 of 331 patients dying with primary carcinoma of the lung (other than oat cell carcinoma). Venous or lymphatic vascular involvement, or both, was greater frequency of tumor embolization htan epidermoid carcinomas (16.5% vs. 5.8%). In 16 of the 33 cases, diffuse, obliterative intimal fibrosis was found in small arteries affected by the tumor embolization. Enlargement of the right cardiac ventricle was present in 26 of the 33 cases. Dyspnea as the initial symptom or as one of the major symptoms, occurred in 21 patients. The average interval between onset of dyspnea and death was 4 weeks. It is concluded that pulmonary arterial embolization is a relatively frequent complication of primary lung carcinoma, particularly adenocarcinoma, which may lead to the developemnt of pulmonary hypertension and cor pulmonale Dyspnea, occurring de novo in patients with primary lung carcinoma is an ominous sign and should alert the clinician to the possibility of arterial pulmonary tumor embolization.