A 58-year-old asymptomatic man was scheduled to undergo FDG PET for whole-body cancer screening. Intense, focally linear F-18 FDG uptake was found in the lower buttock. A malignant tumor in the rectum was suspected. However, physical examinations and a rectocolonscopic study were negative. MRI showing an anorectal fistula provided a morphologic diagnosis. Apparent symptoms and signs of perianal abscess appeared 1 month later. At surgery, an anorectal fistula with actinomycosis and abscess formation were found. FDG PET provided information on the functional status of the fistula. An infectious focus in the anorectal region should be kept in mind and carefully considered when FDG PET is used for tumor imaging.