BACKGROUND: Healthy People 2010 underscores the relevance of eliminating health disparities. Thus, it is paramount to create interventions that promote health for all individuals. PURPOSE: This study examined differences in rates of and reasons for ineligibility among non-Hispanic blacks and whites in a randomized controlled physical activity intervention study. METHODS: Participants (1245 adults) responded to community advertising for the research study. Eligibility at the four pre-randomization assessment sessions was determined by self-reported medical information, resting EKG, 7-Day Physical Activity Recall, fitness test and Stage of Change. We used t-tests to examine the rates of eligibility among participant subgroups. RESULTS: Blacks had higher rates of overall ineligibility (86.9%) than whites (75.1%; p < 0.01) and were more likely to be ineligible due to lack of interest or no-show at a pre-randomization appointment (35.4% vs. 24.3%; p < 0.01). Blacks were more likely to be ineligible for medical reasons after the telephone screen (16.3% vs. 7.8%; p = 0.01). LIMITATIONS: This study did not use a random sampling of potential participants from each of the racial/ethnic groups and thus, there is the potential for selection bias. CONCLUSIONS: Blacks were more likely to choose not to enroll in the study due to a lack of interest, but had similar rates of overall medical ineligibility to whites. This highlights the importance of strategies that enhance interest among blacks, who initially respond to recruitment advertising.