A prospective study of closed intramedullary fixation of fractures of the shaft of the humerus, using Ender nails, was performed over a six-year period. Eighty-nine fractures in eighty-eight patients were treated with no immobilization postoperatively. Three patients were lost to follow-up. Eighty-five of the remaining eighty-six fractures healed, the average time to clinical union being 7.2 weeks. Non-union of one fracture occurred and there were no infections or malunions. Six of the nine preoperative and two postoperative radial-nerve palsies were lesions in continuity and healed spontaneously. The remaining three radial nerves that had been severed by a missile needed further attention. One of the nails backed out in eight patients, requiring revision in five. The average lack of complete extension of the elbow was 4 degrees and flexion of the elbow was 132 degrees. Abduction of the shoulder averaged 91 degrees; external rotation, 54 degrees; and internal rotation, 68 degrees. We conclude that closed intramedullary Ender nailing can be performed safely and effectively in selected fractures of the humeral shaft. However, only fractures that are recalcitrant to closed reduction and immobilization or fractures in the non-compliant patient should be considered for this form of operative treatment.