We report a case of early gastric cancer and early colon cancer detected by positron emission tomography (PET) cancer screening. A 64-year-old male patient with an unremarkable past history except for hypertension and cerebrovascular disease underwent 18F-FDG PET for cancer screening. Images revealed increased uptake in the gastric antrum and sigmoid colon. Both areas appeared suspicious for neoplasm on subsequent fluoroscopy and endoscopy, and biopsies were positive for neoplasia at both sites. The gastric lesion was treated by distal gastrectomy and D2 lymphadenectomy and the colon cancer by endoscopic mucosal resection (EMR). Both surgical specimens were positive for cancer.