Results of surgery to correct fixed flexion contracture of the knee and improve voluntary knee extension in 39 knees in 20 children (11 females, 9 males; mean age 12 years 8 months, age range 5 to 20 years) with cerebral palsy were analyzed. All patients had neuromotor disease and 18 children had spastic diplegia or quadriplegia. All patients could initiate voluntary knee extension but lacked full passive extension. Five patients (10 knees) were free walkers with a mean motor severity index of 19 and mean fixed knee contracture of 20 degrees. Fifteen patients (29 knees) were not free walkers and 13 were wheelchair ambulators. They had a mean motor severity index of 8 and mean fixed knee contractures of 30 degrees. Surgical procedures included various combinations of hamstring lengthening and/or posterior capsulotomy to allow free passive knee extension, with or without quadriceps mechanism shortening, to enhance voluntary extension. The best results were in patients who had hamstring lengthening, posterior capsulotomy, and quadriceps mechanism shortening.