We describe a patient wih subacute cor pulmonale caused by tumor emboli in the lungs. A 64-year-old female suffering from a subacute progressive cough and shortness of breathing died of severe pulmonary hypertension seven days after admission. Neither chest CT scans nor lung perfusion scintigraphy showed any abnormal findings. Microscopic examination after an autopsy revealed diffuse intravascular tumor emboli occluding not only the small pulmonary arteries and arterioles, but also the lymphatic vessels, which were suggested to be metastases of a breast carcinoma resected five years previously. Thus, pulmonary tumor embolism should be considered in the differential diagnosis of primary pulmonary hypertension, particularly in patients with a past history of cancers.