The term "floating knee" describes the flail joint resulting from fractures of the shaft or adjacent metaphyseal region of the ipsilateral femur and tibia; we have reviewed 15 children with this combined injury seen at the Winnipeg Children's Hospital. All had been involved in motor vehicle accidents, eight having been struck while cycling; the biomechanics of such an injury have also been studied. Treatment is difficult and the regimes varied. Results were poor when both fractures were treated nonoperatively and we recommend that at least one of the fractures should be rigidly fixed.