The prevalence and risk of foster care involvement among children with autism spectrum disorder (ASD) relative to children with intellectual disability (ID), children with ASD and ID, and typically developing children were examined using 2001-2007 Medicaid data. Children were followed up to the first foster care placement or until the end of 2007; a discrete time logistic regression analysis was conducted. Both the prevalence and risk of foster care involvement were greatest for children with ASD, and the prevalence increased substantially over the study period among children with ASD. Continued examination of the factors contributing to the higher risk of foster placement is warranted to unravel the complex circumstances facing these vulnerable children and their families.