Between August 1991 and December 1993, 48 fractures of the humeral shaft were treated operatively by retrograde interlocked nailing and followed up prospectively. The fractures were fresh in 39 patients and pathological in 6; in 3 the fracture had already been operated on. The Russell-Taylor nail was always inserted. In 1 of the 39 fresh fractures (2.6%) a postoperative palsy of the radial nerve was observed. In 3 cases (7.8%) comminution occurred at the fracture site. The comminution did not affect fracture healing. Consolidation was seen after an average of 13.7 weeks. In 2 patients a secondary intervention was necessary because of delayed fracture healing. After consolidation of the fractures, the shoulder function was excellent in 92.3% and elbow function excellent in 87.1% of cases. All 6 patients with pathological fractures were free of pain and had acceptable functioning of their stabilized upper arm. In 2 of the 3 fractures that had been operated on before nailing, pseudarthrosis developed. This was attributed to lack of interfragmentary compression. Retrograde nailing of the humeral shaft is a reliable procedure for fresh humeral shaft fractures and for pathological fractures. Small design changes are necessary to obtain a nail that can be used for the treatment of humeral shaft fractures with a lowered healing tendency.