The First Year Inventory is a parent-report measure designed to identify 12-month-old infants at risk for autism spectrum disorder. First Year Inventory taps behaviors that indicate risk in the developmental domains of sensory--regulatory and social--communication functioning. This longitudinal study is a follow-up of 699 children at 3 years of age from a community sample whose parents completed the First Year Inventory when their children were 12 months old. Parents of all 699 children completed the Social Responsiveness Scale-Preschool version and the Developmental Concerns Questionnaire to determine age 3 developmental outcomes. In addition, children deemed at risk for autism spectrum disorder based on liberal cut points on the First Year Inventory, Social Responsiveness Scale-Preschool, and/or Developmental Concerns Questionnaire were invited for in-person diagnostic evaluations. We found 9 children who had a confirmed diagnosis of autism spectrum disorder from the sample of 699. Receiver operating characteristic analyses determined that a two-domain cutoff score yielded optimal classification of children: 31% of those meeting algorithm cutoffs had autism spectrum disorder and 85% had a developmental disability or concern by age 3. These results suggest that the First Year Inventory is a promising tool for identifying 12-month-old infants who are at risk for an eventual diagnosis of autism spectrum disorder.