Retrospective review of records of 26 patients with nonunion of the humeral shaft revealed several factors frequently associated with the development of nonunion. The fractures were transverse and short oblique and treated per primam with hanging casts or open reduction. Surgical fixation was unstable. The types of nonunion were atrophic in 19 patients, hypertrophic in five patients, and synovial pseudarthrosis in two patients. Twenty-four of 26 nonunions (92%) treated with bone grafts and rigid internal fixation healed in an average of 5.6 months. Overall, 47 surgical procedures, including prior procedures, were performed on these 26 nonunions. The average number of operations per patient was 1.8. Successful platings produced immobilization, consisting of an average of 6.8 points of cortical fixation above the nonunion and 7.1 cortices below. Rigid fixation was not obtained in the unsuccessful procedures. Unsuccessful platings were noted to have unstable fixation, with an average of 2.7 points of cortical fixation above the nonunion and 3.0 cortices below. Bone grafting was performed in only 55% of the unsuccessful platings. Optimal treatment of nonunions of the humeral shaft consists of resecting atrophic nonunions, shortening the bones, drilling sclerotic areas, and apposing bleeding diaphyseal surfaces; open reduction with internal fixation with a broad compression plate, including at least six points of cortical fixation above and below the nonunion; compression of the nonunion by means of interfragmentary lag screws, prestressing of the plate, dynamic compression by the plate, or direct compression by the external compression device; and autogeneic cancellous iliac bone grafts.