Sixty-one patients with multiple injuries, which included sixty-three fractures of the humeral diaphysis, were treated by intramedullary stabilization of the fracture with Rush rods or Ender nails. Portals of entry allowing antegrade or retrograde insertion or insertion at the epicondyles were used. For most of the patients, closed intramedullary fixation of the fracture was performed within twenty-four hours of the injury. Adequate follow-up studies were obtained for fifty-six patients (fifty-eight fractures). Stabilization by antegrade insertion gave excellent results if the portal of entry did not violate the rotator cuff. Symptoms of impingement in the shoulder and pain associated with an incorrect position of the portal for antegrade insertion required early removal of the device. Each fracture that was treated with fixation through the epicondylar portal had a poor result, and this technique is not recommended. Retrograde insertion, with the portal of entry located proximal to the olecranon fossa, yielded excellent results. Care must be taken to prevent encroachment on the olecranon fossa, which can result in a block to extension of the elbow. The surgical technique of closed fixation by retrograde insertion is presented. Immediate closed intramedullary stabilization of the fractured humeral shaft resulted in a 94 per cent rate of union and a 62 per cent rate of excellent clinical results. This technique is particularly applicable to patients with multiple trauma, as it minimizes loss of blood and the risk to neurovascular structures while providing stability for mobilization and aggressive pulmonary physiotherapy. In our opinion, however, isolated fractures of the humeral shaft should be treated by non-operative methods.