Background: Femoral lengthening over an intramedullary nail has been described in adults. A technique of femoral lengthening over a humeral intramedullary nail in children is described, and the results and complications are presented. Methods: Nine preadolescent patients (average age, nine years and ten months) with femoral length discrepancy were treated with femoral lengthening over a humeral intramedullary nail. After nail insertion, a monolateral external fixator was placed with half-pins either anterior or posterior to the intramedullary nail, and lengthening was performed through a proximal osteotomy. Results: The femora were lengthened a mean of 6.1 cm (range, 5.0 to 8.0 cm), 19.5% (range, 15.9% to 26.2%) of the preoperative femoral length. Patients had a mean lengthening index of 12.2 days/cm of length (range, 9.5 to 16.9 days/cm of length). Five complications including osteomyelitis, failure of the distal interlocking site, and femoral fracture at the distal end of the nail occurred in four patients; four of the complications led to surgical intervention. No case of proximal femoral valgus secondary to nailing through the greater trochanter had developed by the time of final follow-up. All patients were followed for a minimum of two years postoperatively, with a mean of 128 weeks (range, 111 to 161 weeks). Conclusions: The technique is effective but has a high rate of complications, including osteomyelitis, which developed in two of the nine patients. No avascular necrosis or proximal femoral valgus was noted.