Spica cast immobilization is the preferred treatment for closed femur fracture in preschool children. Flexible intramedullary nailing (FIN) is an alternative treatment but has never been specifically evaluated in this age group. A retrospective analysis of 72 children (mean age, 4.1 years; age range, 1.5-5.9 years) with 73 femur fractures treated by means of FIN was performed. The mean length of stay in hospital was 9.1 days (range, 1-46); weight bearing was started at a mean of 16.4 days (range, 1-60 days) after surgery. Follow-up was available in 62 children (mean, 36.7 months; range, 4-124 months). Complications included early distal nail exteriorization in 9 children (12.3%) during the first 10 years of the study. More than 1 cm of femur overgrowth was noted in 6 children (8.2%). Preschool children with femur fracture treated by means of FIN benefited from short hospital stay, early mobilization and weight bearing. Technique-related complications could be avoided. Long-term follow-up is mandatory in these children.