Rectus femoris transfer to the sartorius is performed in children with cerebral palsy to treat stiff-knee gait. To determine whether preoperative electromyographic (EMG) activity of the rectus femoris is predictive of outcome, we studied 25 children with stiff-knee gait who had preoperative EMG gait analysis before rectus femoris transfer. Fifteen patients had bilateral surgery, and 10 patients had unilateral surgery. The mean age at surgery was 9.6 years for the retrospective review. Patients were divided into three groups based on the recorded EMG patterns of the rectus femoris during the gait cycle. Group I patients had predominant swing-phase activity only. Group II patients had constant rectus activity through the entire gait cycle. Group III patients had normal rectus, defined as minimal EMG activity in the last 75% of swing phase. A repeated gait analysis at a mean of 1.5 years after surgery was available for comparison. In group I, mean peak knee flexion increased 26 degrees after surgery from 44 to 70 degrees. In group II, mean peak knee flexion increased 18 degrees after surgery from 51 to 69 degrees. In group III, mean peak knee flexion increased 12 degrees from 54 to 66 degrees. Results of this study show the greatest improvement in outcome, as measured by knee flexion, occurred in group I in which the rectus fired predominantly in swing phase. Preoperative EMG patterns are therefore useful in determining the outcome after rectus femoris transfer to the sartorius.