Closed intramedullary nailing with or without interlocking screws has been a widely accepted method for the fixation of fractures of the femoral shaft. The design of the interlocking nail introduces the potiential for high stress concentrations at the proximal and distal holes. The authors experienced six cases of metal failure of the interlocking nail in the treatment of fractures of the distal part of the femoral shaft from February 1992 to March 1995. The predisposing factors to fatigue fracture of the interlocking nail were studied. In all patients, the fracture of femur was five centimeters or less from the more proximal of the two distal screw-holes. The risk of fatigue failure may be minimized by using nails that have a larger diameter, by using nails long enough to be driven down to the subchondral area of the knee joint and by avoiding early weight-bearing.