PurposeWe assessed whether an intramedullary lengthening device would reduce the problems normally associated with the external fixation technique. We also wanted to determine whether it is a reliable construct for limb lengthening and deformity correction in the femur.Patients and methods We conducted a matched-pair comparison of 30 femoral lengthenings, 15 with a motorized intramedullary nail (the nail group) and 15 lengthenings with an external ring fixator (the fixator group). The patients were matched based on age, sex, amount of lengthening, and the etiology of leg length discrepancy. Mean lengthening was 35 (25–55) mm in the nail group and 38 (15–75) mm in the fixator group. Outcome measures were: lengthening and alignment achieved, consolidation index, knee range of motion (ROM), and complications.Results The pairs in this matched-pair study were similar in terms of age, sex, diagnosis, and amount of lengthening. The planned amount of lengthening was achieved in all patients in both groups and axis correction was considered sufficient. The mean radiographic consolidation index in the nail group, at 1.5 (0.9–3.0) months/cm, was better than the mean value for the fixator group (1.9 (0.9–3.4) months/cm) (p = 0.01). Knee ROM was better in the nail group during the lengthening, 6 weeks after lengthening was completed, and 6 months after lengthening was completed (p < 0.001). A larger number of complications were observed in the fixator group than in the nail group.Interpretation A lengthening nail may be superior to external fixation in femoral lengthening, when the anatomical conditions and the complexity of the deformity allow the use of an intramedullary nail.