We attempted to quantify the effects of isolated femoral derotation osteotomies using clinical evaluation and gait analysis (kinematics and kinetics) in patients with cerebral palsy (CP). Twelve children with CP were evaluated before and 10 months after isolated femoral derotation osteotomy, and 15 healthy children were evaluated as controls. There were significant improvements on clinical examination. A better position of the hip and ankle in the transverse plane was evident and significant changes occurred in terms of hip and ankle kinetics after surgery. Improvements in kinematics and hip and ankle power are very important biomechanically. The correction of lever arm dysfunction and more physiological hip and ankle power generation result in an improvement in terms of energy consumption, leading to a more functional and economic gait pattern.