Summary:During its 10-year existence, the National Marrow Donor Program (NMDP) has been extremely successful at recruiting potential bone marrow donors to join the volunteer registry. Due in part to successful recruitment and the longevity of the registry, the focus of the NMDP has now shifted to decreasing potential attrition when volunteers are recontacted for additional testing to determine whether they would be the optimal donor for a specific patient. Our own interest in the bone marrow donation process led us to examine four domains of variables -demographic characteristics, volunteer history, recruitment-related characteristics and donationrelated concerns -that we hypothesized would be associated with increased likelihood of donor attrition at a key donor decision-point (DR-stage blood typing). Questionnaires were mailed to potential donors after they were contacted at the DR-stage, and had made the decision of whether or not to continue with blood typing. Our final sample included 756 volunteers who decided to continue with typing, and 258 individuals who declined further participation in the registry. In the bivariate analyses, factors in three of the four domains (all except demographic characteristics) were found to be substantially correlated with likelihood of attrition. Logistic regression indicated that nine central variables across the three domains produced the majority of increased attrition likelihood. Finally, a dose-response analysis suggested that as the number of attrition-related factors endorsed by an individual increased, his/her likelihood of dropping out of the registry also increased. Implications for future research and interventions to reduce potential donor attrition are discussed.