PURPOSE: To evaluate the cause and treatment results of nonunion of humerus shaft fractures. MATERIALS AND METHODS: 39 cases were treated for humerus shaft nonunion between February 1990 and May 2002. The presence of open wounds, initial treatment modality after injury, treatment method for the nonunion and time to union were studied using medical records. The fracture type and appropriateness and problems of the initial surgical treatment were reviewed. Also, Pain and functional recovery in daily living were evaluated in the outpatient clinic, after surgery for nonunion. RESULTS: Amongst the 30 cases, transverse fracture was the most common with 19cases. Most of 29 cases, initially surgically treated, revealed incorrect selection of the internal fixator or technical errors. For surgical treatment of nonunion, open reduction and internal fixation with cancellous bone graft was performed, most commonly, in 36 cases (92.3%). All cases showed clinical and radiographic union at an average of 13.3 weeks. More than 90% of the patients replied minimal pain and excellent functional recovery of daily living at final follow-up. CONCLUSION: If treated with surgery by correct selection of internal fixation methods and accurate technical skills, nonunion incidence can be reduced.