Pulmonary tumor thrombotic microangiopathy (PTTM) is characterized by widespread fibrocellular intimal proliferation of the small pulmonary arteries and arterioles in patients with metastatic carcinoma. Microscopic pulmonary tumor emboli have frequently occurred in patients with malignant tumors; however, few cases of PTTM have been reported. A rare case of a patient with gastric adenocarcinoma who presented with acute dyspnea and lethal respiratory failure is described. Histologically, diffuse fibromuscular intimal thickening causing luminal stenosis and obstruction but containing rather few cancer cells was observed in the small pulmonary arteries and arterioles. These findings were consistent with PTTM. Although PTTM is a rare phenomenon, PTTM should be considered in the differential diagnosis of acute dyspnea or pulmonary hypertension in patients with carcinoma.