A 55-year-old man presented with tumor microembolism manifesting as characteristic patterns of pulmonary perfusion on lung scanning. He had a 2-week history of dyspnea and general fatigue. Echocardiography demonstrated right ventricular enlargement. Computed tomography of the chest was normal. Lung perfusion imaging showed multiple subsegmental peripheral defects, which were characteristic of tumor embolism. Ultrasonography and computed tomography of the abdomen revealed multiple enlargement of the lymph nodes. Upper gastrointestinal panendoscopy showed gastric cancer. At 10 days after admission, he suffered cardiac arrest and died despite resuscitative efforts. Histological examination revealed pulmonary arterial obstruction with tumor cells, and poorly differentiated adenocarcinoma in the stomach and lymph nodes. This case emphasizes the need to include tumor microembolism in the differential diagnosis of dyspnea, even if there is no evidence of an underlying malignant tumor.