PurposeThe Gross Motor Function Measure (GMFM) is a criterion-referenced evaluative measure designed to detect change over time for children diagnosed with cerebral palsy (CP). Reliability of this measure has not been tested for children with osteogenesis imperfecta (OI). The purpose of this study was to determine the intra- and interrater reliabilities of the GMFM for use with children diagnosed with OI.MethodOne physical therapist administered and scored the GMFM for 19 children with OI who were followed at the Shriners Hospital for Children. The assessments were videotaped, then viewed and scored by five physical therapists, including the author, at least six weeks later. Intra- and interrater reliabilities were assessed using intraclass correlation coefficients (ICCs). Kappa statistics were calculated for items demonstrating more disagreement than the majority.ResultsThe ICCs for intrarater reliability of the five dimensions and total score were 0.99. The ICCs for interrater reliability were 0.98 for the lying and rolling dimension and 0.99 for the other dimensions and total score. Kappa statistics for items demonstrating more disagreement than the majority ranged from 0.552 to 1.00.ConclusionsThis study provides evidence of the reliability of the GMFM for children with OI when scored by pediatric physical therapists familiar with the measure. The videotape provided a consistent situation because each therapist did not directly interact with each child, but rather rated a videotaped session of the child’s performance.