The authors prospectively studied three-dimensional kinematics and kinetics of gait in children recovering from a closed, isolated, nonpathologic fracture of the femoral diaphysis, who had been randomly assigned to management by monolateral external fixation or early hip spica casting. The aims were to investigate the gait patterns soon after injury and at 2 years after injury. Children treated with external fixation, walking with the external fixator in situ, demonstrated asymmetric gait abnormalities in all three anatomic planes affecting the trunk, pelvis, hip, knee, and ankle. This appears to be a strategy to minimize movement and pain at the fixator pin sites. The gait pattern normalized rapidly after removal of the external fixator with few kinematic or kinetic abnormalities and no clinically significant disturbances of gait at 2 years after injury. In contrast, children in the early hip spica cast group developed a "crouch gait" pattern in the sagittal plane, most likely due to weakness. They also had abnormal coronal plane kinematics related to shortening of the injured side. Gait patterns improved, but at 2 years there were some persistent gait deviations, probably related to residual limb length discrepancy.