We report the outcome of 19 children aged 5.2 to 13.2 years with 20 fractures of the femoral shaft requiring surgery, who were randomly assigned to have external fixation (EF) or flexible intramedullary nailing (FIN) (10 fractures each). The duration of the operation averaged 56 minutes for the EF group with 1.4 minutes of fluoroscopy, compared with 74 minutes and 2.6 minutes, respectively, for the FIN group. The early postoperative course was similar, but the EF group showed much more callus formation. The time to full weight-bearing, full range of movement and return to school were all shorter in the FIN group. The FIN complications included one transitory foot drop and two cases of bursitis at an insertion site. In the EF group there was one refracture, one rotatory malunion requiring remanipulation and two pin-track infections. At an average follow-up of 14 months two patients in the EF group had mild pain, four had quadriceps wasting, one had leg-length discrepancy of over 1 cm, four had malalignment of over 5°, and one had limited hip rotation. In the FIN group, one patient had mild pain and one had quadriceps wasting; there were no length discrepancies, malalignment or limitation of movement. Parents of the FIN group were more satisfied. We recommend the use of flexible intramedullary nailing for fractures of the femoral shaft which require surgery, and reserve external fixation for open or severely comminuted fractures.