An apparently healthy 46-year-old woman was admitted because of progressive shortness of breath that had begun 2 months before her admission. Physical examination revealed a patient with respiratory distress, tachycardia, and mild jugular venous distention; otherwise, results were unremarkable. Our investigation revealed hypoxia and severe pulmonary hypertension with signs of right heart dysfunction, but no primary cause was found. The patient died 5 days after admission. Autopsy revealed pulmonary lymphangitis carcinomatosis caused by papillary carcinoma. No primary tumor was found.