Infant siblings of children with autism spectrum disorders (ASD) are at elevated risk for social, cognitive, and language delays which may cause parents to become hypervigilant (i.e., excessively worried) about their infant's development. The extent to which parental concern is related to actual cognitive or language impairment in these infants is currently unknown. This study compared the developmental concerns of two groups of parents: those whose infants have an older sibling with ASD (Sibs-ASD) and those whose infants have an older sibling with typical development (Sibs-TD). The association between parental concerns and infant's actual cognitive and language levels was also examined within and across groups. Forty-nine Sibs-ASD and 27 Sibs-TD, ages 12-24 months, participated in this study. Results revealed that parents of Sibs-ASD had higher levels of concern about their infant's development than did parents of Sibs-TD. Furthermore, degree of parental concern was associated with cognitive and language scores, and higher levels of concern were associated with standard scores below clinical cutoffs. Implications for parents and professionals in regard to obtaining specialized assessment and intervention services for Sibs-ASD are discussed. (Contains 2 tables and 1 figure.)