We used the Ilizarov circular external fixator to treat 16 patients with persistent nonunion of the diaphysis of the humerus despite surgical treatment. All patients had pain and severe functional impairment of the affected arm. In ten, nonunion followed intramedullary nailing. We successfully treated these by a closed technique. The nail was left in place and the fracture compressed over it. The fractures of the other six patients had previously been fixed by various methods. We explored these nonunions, removed the fixation devices and excised fibrous tissue and dead bone before stabilising with the Ilizarov fixator. In five patients union was achieved. Bone grafting was not required. In the single patient in whom treatment failed, there had been a severely comminuted open fracture. All except one patient had reduction of pain, and all reported an improvement in function.