To assess the role of stainless steel flexible intramedullary fixation in unstable pediatric femur fractures, we compared a group of 41 stable (transverse or oblique) fractures with a group of 40 unstable (spiral and/or comminuted) fractures treated with stainless steel (Ender) nails placed through a single lateral insertion. The fractures were followed up until clinical and radiographic union was evident with an average follow-up period of 13 months. All fractures were healed at an average of 1.4 months. No infections or refractures occurred. Although minor radiographic angular deformities and shortening were present in both groups, no patient had a clinically detectable angular deformity. Two patients with stable fracture patterns had 10 to 20 degrees of asymmetry in foot progression angles, and 1 patient with an unstable fracture pattern (Winquist grade IV comminution) had a 3-cm limb length difference at final follow-up. Stainless steel flexible intramedullary fixation is effective for unstable pediatric femur fractures if cortical abutment is present.