The authors analyzed 34 cases of humeral shaft fractures in patients who were treated by the Seidel intramedullary nailing from March 1994 to August 1996. Average follow-up period was 18 months(12 months - 26 months). We experienced nonunions of 5 cases(14.7%). The nonunions were found mostly at the comminuted midshaft fractures. The probable cause of nonunion was distraction of fracture ends due to intramedullary endosteal lysis and failure of fanning within postoperative 3 months. These 4 cases of nonunion were treated with rigin internal fixation and additional bone graft, and 1 case with refanning only. So we concluded that the eidel system ws insufficient fixation technique because of the distal spreading-fin loosening. Distal screw-locked nailing seemed to be nn more useful technique unless there are major modifications to the Seidel nail itself.