Of the many causes of dyspnea in patients with cancer, pulmonary tumor embolism is one of the most difficult to diagnose. We report a case of massive pulmonary tumor emboli detected on F-18 FDG PET/CT. PET/CT showed areas of intense accumulation corresponding to the sites of pulmonary emboli detected on contrast-enhanced CT. High serum levels of beta-human chorionic gonadotropin suggested tumor embolism from choriocarcinoma, which was confirmed on autopsy. This case illustrates the usefulness of F-18 FDG PET for differentiating tumor embolism from thromboembolism.