From July 1986 to November 1989, fractures of the humeral shaft in 35 adults who had nonunion of the fracture were managed by plate fixation (19 patients) or antegrade nailing of the fracture fragments with the Seidel interlocking nail (16 patients) together with cancellous bone grafting. The follow-up period ranged from 12 to 52 months. Of the patients who had plate fixation, 89.5% had fracture union within 4.5 +/- 1.7 months; of the patients whose fracture was managed with the Seidel interlocking nail, 87.5% had fracture union within 4.4 +/- 1.8 months. The range of shoulder motion was improved with both techniques. Patients who had plate fixation had more complications than those whose fracture was managed by interlocking nailing (21% v. 12%). The authors prefer interlocking nailing because it is a relatively simpler technique, resulting in fewer complications. It may replace plating in the treatment of nonunion of humeral shaft fractures. However, rotatory instability with interlocking nailing cannot be disregarded, and if this is evident plate fixation should be done.