Why, given similar medical circumstances-high familial risk of breast cancer-will some women elect to join a trial of drugs designed to reduce that risk but others choose not to take part? The aim of this study was to identify measurable differences between women who elect to join a placebo-controlled, double-blind randomised trial of the drug tamoxifen and women who elect not to join. One hundred and six women attending a breast care clinic completed questionnaires covering demographic details, health locus of control, perception of risk and adequacy of medical communication. All were eligible for inclusion in the tamoxifen trial. Only half (n=53) of the sample elected to join, the other half (n=53) declined. Those who declined the trial were significantly more aware of lifestyle factors thought to influence the development of cancers-diet, exercise and oestrogen-prolonging activities (p<0.001), and they also appeared to find the information given by the hospital about tamoxifen harder to understand than did the group who had joined the trial (p=0.01). They could think of significantly fewer positive reasons for joining (p<0.001) and were significantly younger (p=0.001). Participants in both groups significantly overestimated the relative risks of breast cancer. The average estimation of risk for women aged 30 with a family history of breast cancer was 22 times higher than that given by their consultant. This gives rise to considerable concerns about the information underpinning informed consent.