We describe the treatment of an uncommonly late presentation of a recurrent parosteal osteosarcoma of the distal femur. The osteosarcoma had originally been detected 20 years earlier, and had been treated with wide excision and mega-prosthesis to reconstruct the femur. The tumour recurred in close proximity to the femur prosthesis and encased half the femoral stem. Because there was a large piece of metal at the site of recurrence, which might have interfered with computed tomography and magnetic resonance imaging, ultrasonography was used to locate the lesion. The tumour was successfully treated with wide local re-excision. This case emphasises the importance of the long-term follow-up of patients with parosteal osteosarcoma.