States have increasingly transitioned Medicaid enrollees with disabilities from fee-for-service (FFS) to Medicaid Managed Care (MMC), intending to reduce state Medicaid spending and to provide better access to health services. Yet, previous studies on the impact of MMC are limited and findings are inconsistent. We analyzed the impact of MMC on costs by tracking Illinois's Medicaid acute health services expenditures for adults with intellectual and developmental disabilities (IDD) living in the community (n = 1,216) before and after their transition to MMC. Results of the difference-in-differences (DID) regression analysis using an inverse propensity score weight (IPW) matched comparison group (n = 1,134) design suggest that there were no significant state Medicaid cost savings in transitioning people with IDD from FFS to MMC.