Between 1989 and 1994 the health maintenance organization (HMO) share of the Medicare market grew rapidly. It is still heavily concentrated geographically, however. The most influential factor in this growth is HMO penetration into an area's general health care market. Medicare payment rates and prior Medicare HMO penetration also have an impact, but their effects are much weaker. Thus, payment rate reform is likely to have only a modest impact on Medicare HMO growth in the short term. In the longer term, the HMO share of the Medicare market will continue to grow, because HMO penetration in the general health care market is growing rapidly.