This study examined relations between anxiety, aggression, social understanding, IQ, and diagnosis in a sample of 231 children (ages 2-9) diagnosed with Autism Spectrum Disorders (ASDs; Autistic Disorder, Asperger's Disorder, Pervasive Developmental Disorder Not Otherwise Specified) in a hospital setting. Children were administered tests of IQ, and parents completed measures of remaining variables. ASD diagnosis was associated with level of anxiety, and level of IQ explained this relation. IQ was significantly and positively associated with anxiety. Tests of a developmental model to explain the relation between IQ and anxiety showed that social understanding and aggression mediated the relation for toddlers. For preschool- and early elementary school-aged children, respectively, three-way interactions between IQ, social understanding, and aggression predicted anxiety, and graphs of the interactions suggest that the association between IQ and anxiety is increasingly driven by either aggression or social understanding over the course of childhood. (Contains 3 tables and 2 figures.)