We compared the prevalence of various medical and behavioral co-occurring conditions/symptoms between 4- and 8-year-olds with autism spectrum disorder (ASD) from five sites in the Autism and Developmental Disabilities Monitoring Network during the 2010 survey year, accounting for sociodemographic differences. Over 95% of children had at least one co-occurring condition/symptom. Overall, the prevalence was higher in 8- than 4-year-olds for 67% of co-occurring conditions/symptoms examined. Further, our data suggested that co-occurring conditions/symptoms increased or decreased the age at which children were first evaluated for ASD. Similarly, among the 8-year-olds, the prevalence of most co-occurring conditions/symptoms was higher in children with a previous ASD diagnosis documented in their records. These findings are informative for understanding and screening co-occurring conditions/symptoms in ASD.