A case is reported in which fractures of the radius and ulna were fixed with Vitallium plates and screws. Seven years later a painful swelling appeared over the extensor aspect of the forearm. After eight years sarcoma was suspected and a pale tumour infiltrating muscle was found at operation. However, the histology excluded neoplasia and showed massive fibrosis and patchy necrosis of muscle, with chronic inflammatory changes peripherally. After the removal of the metal the swelling disappeared. A sinus down to the ulna followed operation and was not cured two years later. At this stage standard patch testing showed skin sensitivity to cobalt. Metal sensitivity is proposed as the cause of this extraordinary reaction in muscle.