The primary goal of the Prostate, Lung, Colorectal and Ovarian (PLCO) Cancer Screening Trial is to learn whether widespread use of screening tests to detect these cancers will reduce associated mortality. Blacks have the highest age-adjusted cancer incidence and mortality rates of any population group in the United States, but several barriers to their participation in clinical research such as the PLCO trial exist. These barriers involve sociocultural, economic, and individual factors, as well as factors inherent in trial designs. Population diversity in the PLCO trial is necessary to preserve scientific validity and generalizability of trial results. Therefore, the National Cancer Institute and the Centers for Disease Control and Prevention are collaborating to ensure adequate representation of blacks in the PLCO trial. For example, the agencies have funded several new activities designed to better understand and overcome barriers to participation in the trial. These activities include the African American Men Project, a randomized trial designed to evaluate the efficacy of three increasingly intensive recruitment interventions in recruiting black men; the establishment of a minority-focused PLCO trial screening center, a study to identify factors that influenced the decisions of black women recruited to participate in the PLCO trial; and a study to examine the psychosocial factors that influence blacks' decision making to engage in cancer screening and participation in research similar to the PLCO trial. The results of these activities will allow for a more thorough examination of cancer-related issues of importance to blacks and will help shed light on factors that influence their decisions to participate in cancer screening and prevention clinical trials.