What effect does a physician gatekeeper have on costs, access, and quality of care? The researchers evaluated a natural experiment where a managed care plan was offered as a choice for Massachusetts General Hospital employees. Using a non-randomized cohort design, the researchers compared the health experiences of employees and their dependents during the two years prior and the two years after the gatekeeping plan became available. In addition, the researchers analyzed economic, demographic, and other factors that may influence employees to choose a health plan that uses a primary care physician as a gatekeeper. The objective of this study was to provide policymakers with new evidence of the effectiveness of a key component of managed care.