A 47-year-old man was admitted to our hospital because of progressive dyspnea and cough. Physical examination and chest radiographs showed the signs of cor pulmonale. A lung scan using perfused radionuclide revealed multiple peripheral perfusion defects and catheterization of the right heart showed severe pulmonary hypertension. A diagnosis of severe pulmonary embolism was made. Despite intensive care with anti-coagulation therapy, the patient died on the third-hospital day. Autopsy disclosed gastric cancer in the pylorus with metastases to the regional lymph nodes. There were no macroscopic pulmonary artery emboli or parenchymal lesions, but more than 60% of the small arteries and arterioles were occluded by casts of tumor cells. Cor pulmonale due to a pulmonary tumor embolism is a rare complication of cancer. This case is particularly unusual because the embolus-caused cor pulmonale was the initial manifestation of clinically occult, but pathologically advanced, gastric cancer.