The purpose of this study was to determine the effectiveness of muscle-release surgery for children with cerebral palsy (CP) using longitudinal and stratified analysis. Twenty-five children with CP (15 females, 10 males; age range 4 to 16 years; mean age 8 years 2 months, SD 3 years 2 months) were selected from five treatment centres in Japan. Twenty-two children had spastic diplegia, two had spastic quadriplegia, and one had athetospastic quadriplegia. Motor function for each child was assessed using the Gross Motor Function Measure (GMFM). Assessment was conducted on eight separate occasions: 1 month and 1 week before surgery, and 1, 2, 4, 6, 9, and 12 months after surgery. Participants' motor function before surgery was classified using the Gross Motor Function Classification System (GMFCS). Six children were classified at level I, three at level II, six at level III, and 10 at level IV. A significant difference was found after surgery in the GMFCS levels III and IV groups (P < 0.05). Improvement in GMFM scores between 1 week before surgery and 12 months after surgery were 1, 5, 8.5, and 8.5 for GMFCS levels I to IV respectively. Results indicate that this treatment is advantageous for improving motor function in children within GMFCS levels III and IV.