A 65-year-old man without a history of cancer presented to our hospital because he was suspected of having acute pulmonary thromboembolism. Dyspnea that had developed 1 month before admission, had worsened 1-week before admission. Chest computed tomography showed faint ground-glass opacities in the lung fields without filling defects in the pulmonary arteries. He was transferred to the department of respiratory medicine for further investigation. Perfusion scintigraphy showed multiple, small perfusion defects throughout both lungs, and laboratory data showed increased lactic dehydrogenase value and thrombocytopenia. We suspected intravascular lymphoma, and a bone marrow aspiration smear detected malignant cells. We started chemotherapy on a diagnosis of intravascular lymphoma, which resulted in remarkable improvement of respiratory failure and pulmonary hypertension. After that, further evaluation of bone marrow specimen with immunostaining, the malignant cells were found not to be lymphoma cells but cancer cells. The primary site of the cells was not found by further investigation. Because of improvement of oxygenation and pulmonary hypertension, we performed transbronchial lung biopsy and diagnosed pulmonary tumor thrombotic microangiopathy. Here, we report this case and review previous reports.