Between 1982 and 1985, health maintenance organizations (HMO's) entered the Medicare market under the Medicare competition demonstrations. The status and experience of these HMO's, their market areas,, and the benefit packages they offered are presented. Information from case studies of 20 of these HMO's is used to discuss the planning process through which the organizations prepared to enter the Medicare market. Data from administrative reports, submitted by the HMO's, are used to describe the operational experience, including enrollments, utilization, and financial performance.