Abstract Although nonoperative treatment is recognized as an effective treatment method for humeral shaft fractures, it is associated with an approximately 10% risk of nonunion and long-term impairments of the shoulder joint. There is a growing interest to treat even simple humeral shaft fractures operatively to avoid these problems. Intramedullary (IM) nailing has proven to be very effective in the treatment of femoral and tibial shaft fractures and the same method has been adopted for humeral shaft fractures. However, the results regarding union rate and shoulder joint function after antegrade insertion of an IM nail have been very controversial. The purpose of this study was to investigate fracture union, shoulder joint function and symptoms after antegrade IM nailing of humeral shaft fractures, to assess safety and results of IM nailing in pathological fractures, to evaluate the efficacy of exchange nailing and Ilizarov's technique in the treatment of nonunion after IM nailing and to find out, by comparing shoulder joint symptoms and function after antegrade IM nailing and dynamic compression (DC) plate fixation, whether antegrade access to the medullary cavity is the main reason behind shoulder joint problems. During the years 1987-1997, 126 humeral shaft fractures were operated upon in Oulu University Hospital using antegrade IM nailing. The nonunion rate was 22% and distraction of the fracture fragments was the most important risk factor associated with nonunion. The reoperation rate, for various reasons, was 25%. Shoulder joint pain and impairment of function was present in 37% of the patients. In the treatment of 18 pathological fractures IM nailing was a rapid and safe operation, associated with good pain relief. Exchange nailing of 13 cases of nonunion after IM nailing resulted in a union rate of 47% and this method is not useful in the humerus in contrast to tibial and femoral fractures. Permanent nonunion leaves the patient with severe impairment of the shoulder joint and a loose nail may lead to severe osteolysis of cortical bone. In complicated nonunion with poor bone quality, Ilizarov's technique, although associated with a high rate of minor complications and reoperations, worked well. When IM nailing was compared with DC plating it was found that there were no significant differences in shoulder pain, function scores, range-of-motion and strength. Antegrade insertion of the nail, if carried out properly, is probably not the main reason for shoulder joint impairment after IM nailing. Antegrade IM nailing of humeral shaft fractures is associated with several problems, e.g. shoulder joint impairment and difficulties in reconstruction after nonunion, and indications for this method may be exceptional, such as comminuted and pathological fractures.