What is the relationship between Medicare HMO penetration rates and cost, treatment therapies and treatment outcomes for Medicare fee-for-service patients with acute myocardial infarction (AMI)? Applicants at Georgetown University studied "spillover" effects that could be created as Medicare providers change their behaviors to adapt to the changing market environment. The researchers used the Cooperative Cardiovascular Project database, collected for AHCPR, which contains over 140 clinical variables for each patient. These data were linked with hospital level information from HCFA’s Medicare Cost Reports and AHA’s Annual Survey of Hospitals. The American Hospital Association data was also used to develop measures of hospital competition for cardiovascular services in each hospital’s service area. HMO penetration rates were computed for hospital areas using InterStudy, MEDSTAT/Inforum and HCFA data. The objective of the project was to examine the relationships between Medicare managed care penetration and other market factors, and how managed care affects treatment patterns of Medicare patients who are not enrolled in managed care. This study complemented a similar HCFO project being investigated by researchers at Stanford University on health outcomes for cancer patients.