This study evaluated 18F-FDG-PET imaging for the detection of gastrointestinal tract cancer in patients with suspected lung cancer. A total of 351 patients who had abnormal lung shadows and who underwent whole-body FDG-PET between June 1998 and January 2006 were retrospectively entered for analysis. Gastrointestinal tract cancers were subsequently found in 15 patients (4.3%) who had been found to have lung diseases consisting of 7 inflammatory changes, 6 lung cancers, and 2 metastatic lung carcinomas, 9 colon cancers, 4 gastric cancers, and 2 esophageal cancers. Five patients (2 colon cancers, 2 gastric cancers, and 1 esophageal cancer) had early stage carcinoma. In this study, FDG-PET was useful not only for the diagnosis and staging of lung cancer, but also for the detection of unexpected gastrointestinal tract cancers. FDG-PET may be most suitable for cancer screening.