How does growth of HMO market share affect the amount of primary care physicians provide, where they locate, and whether they reduce the amount of patient care they provide? Using data from the American Medical Association, InterStudy and the Area Resource File, this study examined individual physician responses to HMO market penetration, and specifically whether: 1) physicians provide more primary care as HMO penetration increases, 2) specialists in areas with higher HMO market penetration relocate, reduce their clinical practice time, or stop providing patient care altogether; and 3) new physicians locate in areas with higher or lower HMO market share. The researchers also examined how the aggregate amount of primary care and specialty care changes as HMO market penetration changes within different market areas. The objective of the study was to examine how HMO growth in an area affects the overall supply and geographical distribution of physician primary care and specialty care services.