Recruitment and retention of subjects is critically important for the conduct of randomized clinical trials. When significant effort is given toward recruiting members of ethnic minorities in a sample, a similar rate of retention for all subgroups of subjects is essential. Little is known about why research subjects withdraw consent to participate in research and whether attrition is influenced by ethnicity or other factors. The purpose of this study was to determine the reasons for withdrawal from a large, multicenter randomized trial and whether these reasons differed among ethnic and gender subgroups. Additionally, we were interested in comparing withdrawn subjects to matched subjects who stayed in the trial to determine what factors encouraged full study completion. Using a cohort survey design, adult subjects (n=35) who withdrew from a large, multicenter randomized trial and matched subjects who completed the study (n=35) were interviewed by telephone after study completion. Subjects who withdrew consent tended to be female and members of ethnic minorities. The most frequent problems—reported significantly more often by subjects who withdrew consent compared to matched retained subjects—were interference with work, lack of time, complicated and cumbersome record-keeping requirements, difficult study medicine regimens, and difficulty rescheduling appointments due to lack of flexibility on the part of study personnel. Only 17% of withdrawn subjects were satisfied with the overall research experience compared to 52% of matched retained subjects. Matched subjects who completed all required visits in the randomized controlled trial reported three reasons that promoted completion: remuneration, commitment to finish, and belief that the study was important.