How does utilization review affects costs, outcomes, and quality of care? Researchers conducted a natural experiment created by UnitedHealthCare's (UHC) decision to drop utilization review in November 1999. They examined claims and administrative data from the nine UHC regions before and after they terminated the utilization review program. The study included individuals enrolled in the UHC's HMO and PPO plans between October 1997 and February 2001, and will include commercial, Medicare, and Medicaid beneficiaries. Two groups were examined: those with ambulatory care sensitive conditions and patients with diabetes. This study informs decision makers about the extent to which utilization review programs generate savings and affect health outcomes and health care quality.