Shaft fractures of femur and tibia can be treated successfully by intramedullary nailing. In recent years the use of interlocking nails widened the indication for nailing fractures of the proximal and distal bone and more difficult fractures. There are still limits in polytrauma patients, chain fractures with or without participation of joints and in fractures with severe soft tissue injury (open or closed). In these cases primary treatment with external fixation has proved worthwhile. The main problems and risks of primary nailing osteosynthesis occur in the early phase of treatment, whereas complications in external fixation are more likely to occur in later phases of treatment. In changing from external fixation to intramedullary nailing one can see the advantages of both methods. In the literature only small studies have been made mostly with patient groups below 50 in number. The change of method was rarely standardised and the time period between procedures was either late (more than 3 weeks) or arbitrary. The advantages and disadvantages as well as the risk in changing methods are controversial. In a 2-year prospective study from August 1989 to July 1991, patients with II and III degree open and closed femur and tibia fractures as well as trauma patients with fractures were initially treated by external fixation. A change of method from external fixation to intramedullary nailing was performed at the earliest possible time under exact criteria. 61 femur and 106 tibia fractures were accordingly treated and followed. In comparison to early studies there were no differences in bone healing or in functional results. The infection rate in tibia fractures was 1.9% (2 of 106); no infection was seen in femur fractures. The contamination rate at the time of method change was substantially higher at 14%. The difference between contamination and infection rate can be explained by experience in the technique of nailing, considering biological aspects (no or little reaming), the standardized change of method and the prophylactic use of antibiotics. In a follow-up of additional 37 femur and 58 tibia fractures that were treated accordingly, a total infection rate of 1.9% was achieved (2% in femur fractures [n = 98], 1.8% in tibia fractures [n = 164], 1.9% total [n = 262]). In femur and tibia fractures with open or closed soft tissue damage and in multiple trauma patients the treatment with initial stabilisation by external fixation and secondary change to intramedullary nailing can be recommended under certain conditions.