AIM The aim of this study was to determine the agreement and reliability of parent report using a lay version of the Gross Motor Function Classification System (GMFCS) among children with cerebral palsy in the two youngest age bands. METHOD Data were obtained from the Canadian section of the Movement and Participation in LifeActivities of Young Children study database. One hundred and thirty-two parents of two groups of children participated: children aged 2 to 4 years (35 males, 26 females; mean age 3y 2mo; SD 5mo) and children aged 4 to 6 years (39 males, 32 females; mean age 4y 11mo; SD 6mo) at the final data collection point. Therapists classified motor function using the GMFCS and parents used the GMFCS Family Report Questionnaire, with parents and therapists being masked to the others' responses. Agreement between respondents was determined using precise agreement and Cohen's unweighted kappa statistic. Reliability between respondents was determined using the intraclass correlation coefficient (ICC).RESULTS Overall, precise agreement was 77%, chance-corrected agreement was j=0.70 (95% confidence interval [CI] 0.61-0.79), and reliability was ICC=0.95 (95% CI 0.93-0.96).INTERPRETATION These values indicate substantial agreement and reliability between parents of children aged 2 to 6 years and therapists. Some parents had a tendency to rate their children as more functionally limited than did therapists, leading us to question whose the true criterion standard' rating should be.Cerebral palsy (CP) refers to a group of permanent, non-progressive disorders of development of motor function affecting movement and posture in addition to causing activity limitation.1 The Gross Motor Function Classification System (GMFCS) was first described by Palisano et al. 2 to classify the motor function of children with CP. An expanded version that includes older age bands and revision of some details was subsequently published.3 Differences in motor function are reflected in the five classification levels of the GMFCS. Children classified in level I can walk without restrictions but they lack more advanced gross motor abilities. Those in level V are limited in self-mobility, even with the use of assistive technology. 2 Children are assigned a level based on their functional capabilities and typical performance according to specific age bands (<2y, 2-4y, 4-6y, 6-12y, 12-18y The GMFCS is widely used among healthcare professionals to determine clinical decisions involving future rehabilitation, equipment needs, and functional mobility goals. Typically, the parent or guardian has more insight into their child's overall abilities because they view the child in various environments over a longer period of time.11 Accordingly, it is important to determine the level of agreement between healthcare professionals and parents when classifying a child with CP using versions of the GMFCS.Reliability of family report using the GMFCS has been established for children aged between 6 and 12 years (intraclass correlation coefficient [ICC]=0....