This prospective longitudinal multicenter study of ambulatory children with cerebral palsy (CP) examined changes in outcome tool score over time, tool responsiveness, and used a systematic method for defining minimum clinically important differences (MCIDs). Three hundred and eighty-one participants with CP (Gross Motor Function Classification System [GMFCS] Levels I-III; age range 4-18y, mean age 11y [SD 4y 4mo]; 265 diplegia, 116 hemiplegia; 230 males, 151 females). At baseline and follow-up at least 1 year later, Functional Assessment Questionnaire, Gross Motor Function Measure, Pediatric Quality of Life Inventory, Pediatric Outcomes Data Collection Instrument, Pediatric Functional Independence Measure, temporal-spatial gait parameters, and oxygen cost were collected. Adjusted standardized response means determined tool responsiveness for nonsurgical (n=292) and surgical (n=87) groups at GMFCS Levels I to III. Most scores reaching medium or large effect sizes were for GMFCS Level III. Nonsurgical group change scores were used to calculate MCID thresholds for ambulatory children with CP. These values were verified by examining participants who changed GMFCS levels. Tools measuring function were responsive when a change large enough to cause a change in GMFCS level occurred. MCID thresholds assess change in study populations over time, and serve as the basis for designing prospective intervention studies.