This case involved a 38-year-old man who was referred to our hospital with general fatigue, appetite loss, weight loss, cough and exertional dyspnea. Within a couple of days, he was admitted due to advanced dyspnea and general fatigue. Severe hypoxemia was identified and acute right heart failure developed on admission. Treatment was initiated using oxygen, antibiotics and heparin sodium, but the patient died of sudden cardiopulmonary arrest 30 h after admission. Autopsy revealed advanced gastric cancer and widespread tumor embolism together with fibrocellular intimal proliferation and thrombus formation in the small arteries. Pulmonary tumor thrombotic microangiopathy (PTTM) with gastric cancer was diagnosed. PTTM is characterized by widespread fibrocellular intimal proliferation of the small pulmonary arteries and arterioles in patients with metastatic carcinoma. Microscopic pulmonary tumor emboli frequently occur in patients with malignant tumors, but very few cases of PTTM have been reported. PTTM should be considered in the differential diagnosis of acute dyspnea or pulmonary hypertension. In cases of acute cor pulmonal, the existence of malignant cells can be examined using pulmonary arterial wedge aspiration cytology where feasible, in addition to positron emission tomography with F-2-deoxy-2-fluoro-D-glucose, which can be used to investigate certain primary tumors and associated metastatic disease. The suitability of gastroendoscopy to screen for malignancies should be examined.