The First Year Inventory (FYI) is a parent questionnaire designed to assess behaviors in 12-month-olds that suggest risk for an eventual diagnosis of autism. We examined the construct validity of the FYI by comparing retrospective responses of parents of preschool children with autism spectrum disorders (ASD; n = 38), other developmental disabilities (DD; n = 15), and typical development (TD; n = 40). Children with ASD were rated at significantly higher risk on the FYI than children with DD or TD. The DD group was at intermediate risk, also significantly higher than the TD group. These retrospective data strengthen the validity of the FYI and have implications for refining the FYI to improve its utility for prospective screening of 12-month-olds.