A 53-year-old woman was admitted to our hospital due to a severe respiratory condition and malnutrition. Radiological and electrophysiological findings suggested the existence of inexplicable cor pulmonale. Although we commenced to determine the causes of her severe condition, she suddenly died 3 days after admission. Postmortem autopsy revealed tumor cell microemboli in the small pulmonary arteries due to gastric cancer. Such a case of cor pulmonale as the first clinical manifestation is exceptionally rare. Occult malignancy should be considered as a differential diagnosis when one encounters a patient with subacutely aggravated respiratory condition and inexplicable cor pulmonale.