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The purpose of this study was to examine group differences and relationships with later developmental level and autism symptoms using a new clinical tool developed to measure repetitive and stereotyped movements (RSM) in young children.Videotaped behavior samples using the Communication and Symbolic Behavior Scales Developmental Profile (CSBS; Wetherby & Prizant, 2002) were coded for children with autism spectrum disorders (ASD; n=50), developmental delays without ASD (DD; n=25), and typical development (TD; n=50) between 18 and 24 months of age.Children with ASD demonstrated significantly higher rate and larger inventory of RSM with objects and body during a systematic behavior sample than both the DD and TD groups. Measures of RSM were related to concurrent measures of social communication and predicted developmental outcomes and autism symptoms in the fourth year for the ASD group. None of the correlations between RSM and autism symptoms remained significant when controlling for CSBS Symbolic level. RSM with objects predicted unique variance in the severity of autism symptoms in the fourth year beyond that predicted by social communication measures alone.This study provides support for the diagnostic significance of RSM in children under 24 months of age and documents the utility of this RSM measurement tool as a companion to the CSBS.