BACKGROUND Flexible intramedullary nailing (FIMN) is relatively contraindicated for pediatric length unstable femoral fractures. AIM To evaluate FIMN treatment outcomes for pediatric diaphyseal length unstable femoral fractures in patients aged 5 to 13 years. METHODS This retrospective study includes pediatric patients (age range 5-13 years) who received operative treatment for a diaphyseal femoral fracture at a single institution between 2013 and 2019. Length unstable femur fractures treated with FIMN were compared to treatment with other fixation methods [locked intramedullary nailing (IMN), submuscular plating (SMP), and external fixation] and to length stable fractures treated with FIMN. Exclusion criteria included patients who had an underlying predisposition for fractures (e.g., pathologic fractures or osteogenesis imperfecta), polytrauma necessitating intensive care unit care and/or extensive management of other injuries, incomplete records, or no follow-up visits. Patients who had a length stable femoral fracture treated with modalities other than FIMN were excluded as well. RESULTS Ninety-five fractures from ninety-two patients were included in the study and consists of three groups. These three groups are length unstable fractures treated with FIMN (n = 21), length stable fractures treated with FIMN (n = 45), and length unstable fractures treated with either locked IMN, SMP, or external fixator (n = 29). P values < 0.05 were considered statistically significant. Patient characteristic differences that were statistically significant between the groups, length unstable with FIMN and length unstable with locked IMN, SMP, or external fixator, were average age (7.4 years vs 9.3 years, respectively), estimated blood loss (29.2 mL vs 98 mL, respectively) and body mass (27.8 kg vs 35.1 kg, respectively). All other patient characteristic differences were statistically insignificant. Regarding complications, length unstable with FIMN had 9 total complications while length unstable with locked IMN, SMP, or external fixator had 10. Grouping these complications into minor or major, length unstable with locked IMN, SMP, or external fixator had 6 major complication while length unstable with FIMN had 0 major complications. This difference in major complications was statistically significant. Lastly, when comparing patient characteristics between the groups, length unstable with FIMN and length stable with FIMN, all characteristics were statistically similar except time to weight bearing (39 d vs 29 d respectively). When analyzing complication differences between these two groups (9 total complications, 0 major vs 20 total complications, 4 major), the complication rates were considered statistically similar. CONCLUSION FIMN is effective for length unstable fractures, having a low rate of complications. FIMN is a suitable option for length stable and length unstable femur fractures alike.