Dynamic intramedullary fixation depends on the configuration of the fracture for postoperative stability. Unanticipated loss of reduction of the fracture after dynamic intramedullary nailing of the femur may result from errors in surgical decision-making, specifically the failure to insert both proximal and distal interlocking screws. Of 133 dynamic femoral intramedullary nailings that were performed after interlocking techniques became routinely available, fourteen (10.5 per cent) were complicated by loss of postoperative fixation and reduction. Thirteen of the fourteen femora shortened an average of 2.0 centimeters; the remaining femur shortened slightly, with clinical loss of rotational stability. Eight of fourteen patients elected some form of surgical revision, most commonly closed osteoclasis with restoration of femoral length, followed by the insertion of a statically locked nail. Errors in surgical judgment were attributed to inadequate preoperative analysis of the pattern of the fracture; undetected intraoperative comminution during reaming or insertion of the nail, or both; or postoperative failure to recognize an increase in comminution and instability of the fracture. We suggest using high-quality preoperative radiographs to detect non-displaced comminution of the major fracture fragments. Any increase in comminution of the fracture that occurs with reaming of the canal or insertion of the nail is an indication for static interlocking fixation. Radiographs that are made immediately postoperatively should be analyzed while the patient is under anesthesia, and any previously undetected instability of the fracture should be treated by static interlocking fixation. Dynamic intramedullary stabilization of the femur should be reserved for transverse or short oblique fractures at the femoral isthmus that have type-I or type-II comminution.