Statement of purpose of study: To determine how effective Flexible Intramedullary Nails are in treating tibial and femoral fractures in adolescents. Summary of Methods used: Retrospective review of consecutive adolescent patients treated over a seven year period with Flexible Intramedullary Nails for tibial and femoral fractures. Statement of Conclusion: We conclude that the higher than expected rates of malunion and delayed union suggest that other treatments should be considered when treating adolescents with unstable tibial or femoral fractures. Introduction Flexible intramedullary nails (FIN) are increasingly used in the management of paediatric tibial and femoral fractures. Recently, concerns have been raised regarding the use of FIN in older children. The aim of this study was to determine how effective FIN's are in treating tibial and femoral fractures in adolescents. Methods Hospital records were used to identify all patients aged 11 years or older under going FIN for tibial and femoral fractures between 2003 and 2009. Radiographs and case notes were reviewed to identify complications. Results 35 consecutive adolescent patients underwent FIN for tibial (n=21) and femoral fractures (n=15) with a mean age of 12.9 years. 2 femoral and 9 tibial fractures were open. Eight patients sustained multiple injuries. Mean radiographic follow up was 29 weeks. 60% (n=9) and 38% (n=9) of femoral and tibial fractures respectively malunited. Fracture severity was associated with increased malunion for both tibial and femoral fractures (P=0.046 and P=0.044 respectively). There were no cases of non-union. 2 femoral fractures took longer than 20 weeks to unite and 7 tibial fractures took longer than 16 weeks to unite. One patient developed post operative compartment syndrome, one patient developed deep infection and two patients were treated with post operative traction for loss of fracture position. Discussion Previous publications from multiple centres, including ours, have demonstrated excellent results of FIN for tibial and femoral fractures in the general paediatric population. However, concerns have recently been raised about the use of FIN in older, heavier children and with unstable fracture patterns. This is the first published series of adolescent patients undergoing FIN. We conclude that the higher than expected rates of malunion and delayed union suggest that other treatments should be considered when treating adolescents with unstable tibial or femoral fractures.