Florid reactive periostitis ossificans is a rare benign lesion usually affecting the tubular bones of the hands and feet, and its histological features may be confused with those of infection and osteosarcoma. We report a case with florid reactive periostitis ossificans of the femur showing increased tracer uptake on both Tc-MDP bone scan and F-FDG PET/CT mimicking a local recurrence in a 15-year-old patient with high-grade osteosarcoma.