Objective: Examine the influences of race, socioeconomic status, sex, and age on barriers to participation in a study of cross-sectional differences and longitudinal changes in health-related outcomes. Methods: We designed a multidisciplinary, community-based, prospective longitudinal epidemiologic study among socioeconomically diverse African Americans and Whites. We recruited 3722 participants from Baltimore, Md. with a mean age of 47.7 (range 30-64) years, 45%males; 2200 African Americans (59%) and 1522 whites (41%); 41% reported household incomes below the 125%poverty delimiter. Results: There were no significant age differences associated with sex or race. Participants below the 125% poverty delimiter were slightly younger than those above the delimiter. Age, race, and sex, but not poverty status, were associated with the likelihood of a physical examination. Older participants, women, and Whites were more likely to complete their examinations. Among those who completed their examinations, there were no age differences associated with sex and poverty status, but African Americans were negligibly younger than Whites. Conclusions: Although some literature suggests that minorities and low-income people are less willing to participate in clinical research, these baseline data suggest that African Americans individuals and individuals from households with incomes below 125% of the poverty level are at least as willing to participate in observational clinical studies as Whites and higher income individuals of similar age and sex.