This study aims to (a) examine the roles of knowledge, distrust in medical professionals, information sources, and 2 dimensions of religiosity (i.e., religious activity and religious belief) in influencing willingness to participate (WTP) in cancer clinical trials and to (b) compare the results for Caucasians and African Americans in order to inform future recruitment. An online survey was fielded via a Knowledge Networks panel with a nationally representative sample including 478 Caucasians and 173 African Americans. The results showed that distrust in medical professionals was a strong barrier to WTP for both ethnic groups, whereas factual knowledge about trial procedures was not associated with WTP for either ethnic group. Seeking trial information from doctors was positively associated with WTP for Caucasians; seeking trial information from hospitals was positively associated with WTP for African Americans. More interestingly, levels of religious activity negatively predicted WTP for Caucasians but positively predicted WTP for African Americans. Self-reported religious belief was not associated with WTP for either ethnic group. In sum, although distrust is a common barrier to WTP, the influence of preferred information sources and religious activity on WTP varies as a function of ethnicity.