From January 1992 to June 1997, we reviewed retrospectively eighteen femoral shaft nonunions after intramedullary nailing and followed for an average of 31(18-53)months. We investigated causes of nonunion and analyzed the results according to operative method. Nonunion was divided into infected(4 cases) or noninfected types(14 cases). The causes of noninfected nonunion were insufficient stability(7 cases) and bone defects(4 cases). In insufficient stability, there were loosening of locked screw in four, absence of locked screw in two, and breakage of locked screw in one. There was a correlation between severe comminuted fracture with an early weight bearing and screw failure(p<0.05). Seven cases of nonunion were treated with compression plate and cancellous bone grafting, four cases only bone grafting, three cases Ilizarov external fixations, three cases dynamization, and one case renailing. The clinical and roentgenographic healing processes were recorded. All achieved solid unions within an average period of 11.7(2.5-41)months. An average time to union was 5.2(3-7)months after dynamization, 7.3(7-8)months after bone grafting, 12 months after renailing, 12.9(4-25)months after compression plate and cancellous bone grafting, and 21.3(11-41)months after Ilizarov external fixation. The union period with noninfected nonunion was significantly shorter than infected nonunion(9.2 months vs. 20.3 months; p<0.05).