Objectives: To examine different clinically relevant eligibility criteria sets to determine how they differ in numbers and characteristics of individuals served. Design: Cross-sectional analysis of the 2000 wave of the Health and Retirement Study (HRS), a nationally representative longitudinal health interview survey of adults aged 50 and older. Setting: Population-based cohort of community-dwelling older adults, subset of an ongoing longitudinal health interview survey. Participants: Adults aged 65 and older who were respondents in the 2000 wave of the HRS (n=10,640, representing approximately 33.6 million Medicare beneficiaries). Measurements: Three clinical criteria sets were examined that included different combinations of medical conditions, cognitive impairment, and activity of daily living/instrumental activity of daily living (ADL/IADL) dependency. Results: A small portion of Medicare beneficiaries (1.3?5.8%) would be eligible for care coordination, depending on the criteria set chosen. A criteria set recently proposed by Congress (at least four severe complex medical conditions and one ADL or IADL dependency) would apply to 427,000 adults aged 65 and older in the United States. Criteria emphasizing cognitive impairment would serve an older population. Conclusion: Several criteria sets for a Medicare care-coordination benefit are clinically reasonable, but different definitions of eligibility would serve different numbers and population groups of older adults.