Summary: Fifty-five children with 55 fractures of the femur were prospectively studied for the presence of ligamentous instability of the knee within 3 weeks from the time of injury. Two patients (4%) demonstrated instability on examination under anesthesia. A 9-year, 4-month-old boy sustained a second-degree injury to the lateral collateral ligament that healed after hip spica cast treatment. The other patient, a 12-year, 8-month-old boy, sustained an avulsion fracture of the posterior cruciate ligament from the tibial insertion with >1 cm posterior displacement of the tibia on the posterior drawer test. After placement of an intramedullary rod, the avulsion fracture was treated with open reduction and internal fixation. Although the incidence of ligamentous instability of the knee in this study is less than that reported for adults with femur fractures, children should nevertheless have a thorough knee examination for ligamentous stability as well as radiographic evaluation of the knee.