Purpose: To assess relationships among: 1) interest in tamoxifen (Tam) use for breast cancer chemoprevention, 2) weighing of Tam's risks and benefits, 3) perceptions of breast cancer risk and 4) numeracy. Methods: 56 women recruited from gynecology clinics in North Carolina who qualified for Tam were given their five-year breast cancer risk estimate (Gail score) and a computerized decision aid that provided tailored numerical feedback as to their chances of experiencing five health risks and five health benefits of taking Tam for five years. After reviewing the information, participants were asked: 1) for their perceived lifetime breast cancer risk, 2) whether Tam increased, decreased or did not affect their chances of experiencing the 10 health events, 3) for their overall weighing of Tam's risks and benefits for self (1=benefits outweigh the risks by a lot to 5=risks outweigh the benefits by a lot), and 4) interest in taking Tam (1=not at all to 5=extremely). All women completed Lipkus and colleagues' measure of numeracy. Results: Women with greater numeracy were more likely to specify correctly how Tam affected their chances of experiencing these events (r=.57, p<.0001, M=7 out of 10 correct). Participants viewed Tam's risks as slightly outweighing the benefits (M=3.25). Greater perceived lifetime breast cancer risk and a greater perceived risk-to-benefit ratio was related to more (r=.27, p<.06) and less interest in use (r=-.32, p<.02, M=2.3 in interest), respectively. Numeracy was not related to interest in use. Perceptions of Tam's benefit-to-risk ratio were related primarily to actual risks (r=.34, p<.01 partialling actual benefit) than actual benefits (r=-.11, NS, partialling actual risk), despite our data showing that increasing breast cancer risk (M(Gail) = 2.4%, range 1.67% to 6.7%) was related to an improved benefit-to-risk ratio (r=.34, p<.02) and especially actual benefit (r=.60, p<.0001). Conclusions: These data suggest that more research is needed to understand why women disproportionately consider Tam's risks rather the benefits, especially when Tam's benefit-to-risk ratio improves with greater breast cancer risk. Numeracy may play an important role in how women understand numerical information on Tam. Results may have implications for how medical personnel may wish to present information on Tam's risks and benefits.