Twenty-one cases of non-union of the isolated fracture of the shaft of the ulna were reviewed. The factors which have been shown to predispose to non-union include the site of the fracture, its initial displacement, comminution, the presence of multiple injuries and early mobilization. The use of rigid fixation and the delay of one week in operation reduced the incidence of non-union in those internally fixed. No evidence was found that internal fixation reduced the incidence of delay in union, compared with closed treatment. Fourteen of the fractures later united. Plating the fracture of internal fixation with bone grafting secured union in 11 out of 12 cases. These were the only satisfactory methods of treatment.