Objective: Children with autism spectrum disorders (ASDs) often have co-occurring conditions, but little is known on the effect of those conditions on their medical care cost. Medical expenditures attributable to ASDs among Medicaid-enrolled children were calculated, and the effects of 3 commonly co-occurring conditions—intellectual disability (ID), attention deficit/hyperactivity disorder (ADHD), and epilepsy—on those expenditures were analyzed. Methods: Using MarketScan Medicaid Multi-State Databases (2003–2005) and the International Classification of Disease, Ninth Revision, children with ASD were identified. Children without ASD formed the comparison group. The 3 co-occurring conditions were identified among both the ASD and the comparison groups. Annual mean, median, and 95th percentile of total expenditures were calculated for children with ASD and the co-occurring conditions and compared with those of children without ASD. Multivariate analyses established the influence of each of those co-occurring conditions on the average expenditures for children with and without ASD. Results: In 2005, 47% of children with ASD had at least 1 selected co-occurring condition; attention deficit/hyperactivity disorder was the most common, at 30%. The mean medical expenditures for children with ASD were 6 times higher than those of the comparison group. Children with ASD and ID incurred expenditures 2.7 times higher than did children with ASD and no co-occurring condition. Conclusion: Medicaid-enrolled children with ASD incurred higher medical costs than did Medicaid-enrolled children without ASD. Among Medicaid-enrolled children with ASD, cost varied substantially based on the presence of another neurodevelopmental disorder. In particular, children with ID had much higher costs than did other children with ASD.