Recent policy reforms, notably the 1997 Balanced Budget Act (BBA), the Benefit Improvement and Protection Act (BIPA) of 2000 and the 2003 Medicare Modernization Act (MMA) have sought to expand the role of managed care in Medicare, despite limited information about quality of care provided by Medicare managed care (MMC) plans.  This investigation uses a unique inpatient hospitalization dataset containing over 9 million records to study the effects of Medicare managed care on rates of potentially preventable admissions, an indicator of quality of outpatient care and rates of referral-sensitive procedures indicating access to elective procedures in four states from 1999 - 2004.