A growing number of Medicare beneficiaries are enrolling in Medicare HMOs as an alternative to the traditional Medicare program. However, Medicare HMO enrollment has not grown uniformly in market areas across the country. Case studies conducted by researchers at Mathematica Policy Research, Inc. for The Kaiser Family Foundation describe the evolution of Medicare managed care in the two largest markets in the United States: Los Angeles County and New York City. The reports also discuss the impact of the shift to Medicare managed care on Medicare beneficiaries, plans, and providers in each of the respective markets.