A 31-year-old woman developed polyarthralgia and extension disorder of the right elbow of 2 months' duration without abdominal symptom. Laboratory findings revealed C-reactive protein 2.7 mg/dL. Anti-nuclear antibody (ANA), rheumatoid factor (RF), and anti-cyclic citrullinated peptide (CCP) antibody were negative. F-18 FDG PET/CT showed elevated pinpoint uptakes of FDG in the entheses. In contrast, MRI revealed no specific findings. Four months after PET/CT, she developed right lower abdominal pain. Colonoscopy showed abscess in ascending colon, which the initial PET/CT finding might mean early image of infective colitis. A diagnosis of early reactive arthritis was made. Treatment with antibiotics improved colitis, colonic abscess, and enthesitis after 2 months. Moreover, F-18 FDG PET/CT showed no accumulation in colon and entheses. These findings suggest that F-18 FDG PET/CT scanning allows enthesitis in the early stage of reactive arthritis, before the possible detection by the other procedures.