The Interrater Reliability of the Greek Expanded and Revised Gross Motor Function Classification System and the Family Report Questionnaire in Cerebral Palsy

<b>Background/Objectives</b>: The Gross Motor Function Classification System-Expanded & Revised (GMFCS-E&R) is widely used to describe gross motor performance in children with cerebral palsy (CP). Although Greek-language materials are available, interrater reliability across healthcare professionals and parents has not been examined. This study evaluated the reliability of the Greek GMFCS-E&R among a pediatric neurologist, a pediatric physiotherapist, and parents, with an emphasis on descriptors and illustrations for older children and adolescents. <b>Methods</b>: A cross-sectional study was conducted with 111 children and adolescents with CP aged 2-18 years. Professionals classified each child using the Greek GMFCS-E&R brochure (ages 2-6) or the descriptors and illustrations (ages 6-12 and 12-18). Parents completed the age-appropriate GMFCS Family Report Questionnaire. Agreement among the three raters was assessed using Fleiss' kappa (κ_F), and pairwise agreement using weighted Cohen's kappa (κ_Cw), overall and by age band. <b>Results</b>: Overall interrater reliability was substantial (κ_F = 0.77). Agreement by GMFCS level ranged from κ_F = 0.68 (Level V) to κ_F = 0.85 (Level I). Reliability increased with age, reaching κ_F = 0.74-0.85 in adolescents. Pairwise agreement was excellent across all rater pairs, with near-perfect concordance between the pediatric neurologist and physiotherapist (κ_Cw = 0.98). In >60% of disagreements, parents assigned higher levels, typically between adjacent categories. <b>Conclusions</b>: The Greek-language GMFCS-E&R demonstrates high interrater reliability among healthcare professionals and parents, with excellent agreement when using descriptors and illustrations for older children and adolescents. The GMFCS-FR effectively incorporates parental perspectives and complements clinical assessment, supporting the use of the Greek GMFCS-E&R in routine clinical practice and research settings.