There are good biological reasons for concern about the influence of oral contraceptives and/or hormone replacement therapy (HRT) on the risks of breast and ovarian cancer. Despite the widespread and growing use of exogenous hormones, epidemiological data are deficient, especially in relation to prolonged ( 10 years) use of combined oestrogen/ progestagen preparations. Most studies suggest that the relative risk of breast cancer, in particular, is increased by use of both oral contraceptives and HRT, that risk increases with duration of use and that it may be higher for combined preparations. It is uncertain whether a pre-existing high genetic risk of breast cancer is compounded by use of exogenous hormones. This is a serious issue since a small relative increase in a large risk will have a major effect.