Common peroneal nerve injury may occur during surgery, particularly when patients are placed in lithotomy position. We report a case of common peroneal nerve palsy following a surgical procedure. Incorrect posture of a surgical assistant which made him lean his body against the patient's knee support might possibly be the cause of this injury. The patient reported that she had a left drop foot and a numbness of her left lower extremity following surgery. Electromyographic and nerve conduction studies revealed a left common peroneal nerve palsy. Physical therapy was started immediately. Patient's neurologic function of the leg totally recovered 3 months later.