This study examines whether parental report of social-communicative and repetitive behaviors at 12 months can be helpful in identifying autism spectrum disorder (ASD) in younger siblings of children with ASD [high-risk (HR)-siblings]. Parents of HR-siblings and infants without a family history of ASD completed the First Year Inventory at 12 months. Developmental outcomes were based on 24- or 36-month assessments. HR-siblings later diagnosed with ASD showed greater impairments in social communication than those with other developmental outcomes based on parental and clinician ratings. Parental report of decline in play and communication and impaired vocal imitation correctly classified a majority of ASD cases with high specificity. These preliminary findings have important implications for the development of early screening instruments for ASD in HR-siblings.