Increasing the number of bone marrow (BM) donors is important to ensure sufficient diversity on BM registries to meet the needs of patients. This study used an experimental approach to test the hypothesis that providing information about the risks of BM donation to allay unsubstantiated fears would reduce male and female participants' perceptions of risk for donation and joining the Australian Bone Marrow Donor Registry (ABMDR). Males' and females' intentions to register on the ABMDR and their attitudes, norms and perceived behavioural control (efficacy) in relation to registering were also explored. Participants were allocated randomly to either a risk (exposed to risk information about BM donation) or no-risk (not exposed to risk information) condition. In partial support of hypotheses, exposure to risk information did reduce perceived risk for registering on the ABMDR for males only. Participants in the risk condition also demonstrated lower scores on attitude (males only) and intention compared with participants in the no-risk condition. These findings highlight the complex role of risk perceptions and gender differences in understanding people's decisions to join a BM registry.