Medicaid managed care rates are required to be actuarially sound. A state is required to submit its rate-setting methodology, including a description of the data used, to the Department of Health and Human Services' (HHS) Centers for Medicare & Medicaid Services (CMS) for approval. The Children's Health Insurance Program Reauthorization Act of 2009 required GAO to examine the extent to which states' rates are actuarially sound. GAO assessed CMS oversight of states' compliance with the actuarial soundness requirements and efforts to ensure the quality of data used to set rates. GAO reviewed documents, including rate-setting review files, from 6 of CMS's 10 regional offices. The selected offices oversaw 26 of the 34 states with comprehensive managed care programs; the states' programs varied in size and accounted for over 85 percent of managed care enrollment. GAO interviewed CMS officials and Medicaid officials from 11 states that were chosen based in part on variation in program size and geography. GAO recommends that CMS implement a mechanism to track state compliance with the requirements, clarify guidance on rate-setting reviews, and make use of information on data quality in overseeing states' rate setting.