A parent-report instrument, the First Year Inventory (FYI), was developed to assess behaviors in 12-month-old infants that suggest risk for an eventual diagnosis of autism. The target behaviors were identified from retrospective and prospective studies. FYIs were mailed to 5,941 families and 25% (N = 1,496) were returned, with higher return rates for white families and for families with greater educational attainment. Ad hoc groups of questions afforded measurement of eight specific constructs, which were combined to establish a general risk index. Boys had higher risk scores than did girls. Maternal race and education influenced answers. A small percentage of infants appeared to be at notably elevated risk. Large-scale longitudinal research is warranted to determine whether the FYI can predict an eventual diagnosis of autism.