From 1979 to 1982, 64 femoral shaft fractures in 62 patients were treated by closed interlocking nailing at Harborview Medical Center, Seattle, WA, U.S.A., and Parkland Memorial Hospital, Dallas, TX, U.S.A. Twenty-nine patients sustained multiple system injuries and 29 of the involved extremities (45%) had at least one additional injury. There were 17 (26%) open fractures. Static mode nailing was used to treat 52 fractures; dynamic mode nailing was performed for 12 fractures. Patient follow-up averaged 17 months (range 7-41 months). The average time to union was 13.5 weeks. Normal femoral length within 1 cm was achieved in 97% of cases. Knee range of motion averaged 127 degrees. Complications (9%) included two delayed unions, one nonunion, two cases of shortening or lengthening of more than 1 cm, and one case of malunion with angulation or more than 10 degrees. The delayed unions and nonunions healed after one additional procedure. This study shows that closed interlocking nailing is a safe, effective technique that provides stable fixation in most unstable femoral shaft fractures. This technique represents a major advance in the treatment of difficult femoral shaft fractures that would be poorly suited for standard closed nailing.