An 'anti-oestrogen' such as tamoxifen may protect prophylactically against breast cancer. At the Royal Marsden Hospital, the blind randomised feasibility study of tamoxifen 20 mg per day versus placebo in 200 healthy women has been extended into a pilot trial. A total of 435 women with a family history of breast cancer have been accrued. Compliance, acute toxicity, clotting factors, lipids and bone mass were assessed. The pilot trial has confirmed the findings of the feasibility study. Compliance was high and the frequency of side-effects was similar in both groups, except for a significant increase in hot flushes in the tamoxifen-treated women (33 vs. 17%). Bone mass and clotting factors were not affected. Tamoxifen significantly reduced serum cholesterol, low-density lipoprotein cholesterol (LDLC) and apolipoprotein B levels in post-menopausal women. In premenopausal women, the effects on lipids and lipoproteins was smaller with a significant fall in total serum cholesterol and LDLC only. The trial has approval to accrue up to 1000 women.