The purpose of this study was to determine whether the Gross Motor Function Measure (GMFM-88) is sensitive to within-child changes in function as a result of children who use an ambulatory aid or orthoses in comparison with unaided or barefoot function. Data from 257 children (140 males, 117 females) with cerebral palsy (CP) were analyzed from a 5-year longitudinal study. The children's age ranged from 2 to 15 years (mean 7y 4mo; SD 2y 11mo), and type of CP, included spastic (n=206 [80.2%]),dyskinetic (n=13 [5.1%]),ataxic (n=9 [3.5%]), hypotonic or mixed (n=27 [10.5%]), and those missing (n=2 [0.8%]), and in motor ability (Gross Motor Function Classification System [GMFCS] levels I, n=40, II, n=34, III, n=93, IV, n=76, and V, n=14). Paired t-tests between barefoot and aided assessments showed significantly higher GMFM-88 total scores overall (n=257) for aided assessment and for three subgroups (ankle-foot orthoses only, ambulatory aid only, ankle-foot orthoses plus ambulatory aid), providing evidence that the GMFM-88 is sensitive to functional changes as a result of using an aid and/or orthoses. With regard to changes within the orthoses-only group, significant changes varied by GMFCS level. The GMFM-88 might provide a clinically useful tool to help in understanding the impact of ambulatory aids and orthoses on gross motor skills of children with CP.