BACKGROUND: Lower extremity reconstruction after resection of long bone tumors in children is challenging because of the unique functional demands and growth potential of the lower extremity. The use of a free fibula flap inside a massive bone allograft provides a reliable reconstructive option. The authors evaluate the surgical and functional outcomes of using this technique. METHODS: This is a retrospective review of 12 consecutive patients who underwent reconstruction of segmental femur or tibia defects using a free fibula flap inside a massive bone allograft between 2003 and 2011. Complications and functional outcomes are reported. RESULTS: Twelve patients with a mean age of 15.8 years (range, 3 to 49 years) were included in the study. Eight femur defects and four tibia defects were reconstructed. The mean follow-up time was 41.4 months. Two constructs were removed because of infection, three patients required bone grafting for nonunion, one patient required an additional operation to excise a skin paddle, and one patient experienced a lower extremity deep vein thrombosis. The mean time to achieve full weight bearing was 14.3 months. CONCLUSIONS: The use of a free fibula flap inside a massive bone allograft after bone tumor resection provides an option for lower extremity reconstruction. The allograft component increases the initial strength of the reconstruction, whereas the vascularized fibula component is thought to increase the biologic potential for osteosynthesis and ultimately provide a potentially lifelong durable reconstruction. Patients who achieve oncologic control are likely to enjoy a highly functional long-term outcome. CLINICAL QUESTION/LEVEL OF EVIDENCE: Therapeutic, IV.