Case:An 18-year-old man with 48° of internal malrotation of the femur after nailing underwent derotational osteotomy with gait dynamics and electromyography data collected preoperatively and postoperatively. Hip abduction and internal foot progression angles were significantly deviated from normal preoperatively compared with the contralateral side. At 10 months postoperatively, the hip was abducted and externally rotated throughout the entire gait cycle. His Trendelenburg gait had resolved, and he reported no residual functional concerns. Before corrective osteotomy, walking velocity was significantly slower with shorter stride lengths.Conclusion:Significant internal malrotation of the femur impairs hip abduction and foot progression angles as well as gluteus medius activation during ambulation. Derotational osteotomy considerably corrected these values.