Objective: Many states have implemented regulations (commonly referred to as waivers) to increase access to publicly insured services for autism spectrum disorders (ASD). In recent years, several states have passed legislation requiring improved coverage for ASD services by private insurers. This study examines the impact of such legislation on use of Medicaid-funded ASD services. Method: We used Medicaid claims data from July 1, 2006, through June 30, 2010, to identify children with ASD and to assess their use of behavioral health services and psychotropic medications. Service and medication use were examined in four consecutive 12-month periods: the 2 years preceding passage of the legislation, the year after passage but before implementation, and the year after implementation. We examined differences in use of services and medications, and used growth rates from nonwaiver children to estimate the impact of the legislation on Medicaid spending for waiver-eligible children with ASD. Results: The number of children with ASD receiving Medicaid services increased 20% from 2006-2007 to 2009-2010. The growth rate among children affected by the legislation was comparable to that of other groups before passage of the legislation but decreased after the legislation's passage. We project that, without the legislation, growth in this population would have been 46% greater in 2009-2010 than observed, associated with spending of more than $8 million in 2009-2010. Conclusions: Passage of legislation increasing private insurance coverage of ASD services may decrease the number of families seeking eligibility to obtain Medicaid-funded services, with an associated substantial decrease in Medicaid expenditures. (Contains 3 tables and 1 figure.)