OBJECTIVE: To examine the effect of HMO penetration on physician retirement. STUDY DESIGN: We linked together historical data from the Physician Masterfile of the American Medical Association for successive years to track changes in physicians' activity status between 1980 and 1997. We used a multivariate discrete-time survival model to examine how the probability of physician retirement was affected by the level of HMO penetration in the physician's market area, controlling for other physician and market characteristics.