We studied 75 cases of conventional Kuntscher intramedullary nailing and 52 cases of interlocking intramedullary nailing from July 1980 to October 1988 for femur shaft fractures. The conventional Kuntscher intramedullary nailing was used for fresh fracture and the interlocking intramedullary nailing was used not only fresh fracture, but also unstable fracture, nonunion and pathologic fracture. The conventional Kuntscher intramedullary nailing was frequently used in middle one-third level, but the interlocking intramedullary nailing could be applied widely from subtrochanteric area to supracondylar level in femur. The average operation time in patient who had no associated injury was 3.6 hours in conventional Kuntscher intramedullary nailing and 3.25 hours in interlocking intramedullary nailing. Time period for union was much less in interlocking intramedullary nailing than conventional Kuntscher intramedullary nailing. Some kinds of immobilization was needed shortly in conventional Kuntscher intramedullary nailing but immediate postoperative ambulation was possible in interlocking intramedullary nailing. Interlocking intramedullary nailing can prevent the angulation, shortening and trochanteric bursitis due to migration of the intramedullary nail.