Because endogenous hormones play a major role in the risk of breast, endometrial and ovarian cancer, the impact on risk of oral contraceptives and of hormonal therapy given at about the time of menopause has been a major concern. Numerous studies provide insight into whether risk is increased or decreased in association with use of these preparations. Oral contraceptives present a chemopreventive opportunity for endometrial cancer and ovarian cancer as risk is dramatically lower among women who have used these preparations than among those who have not. Balanced against this is the potential for risk of breast cancer to be increased with oral contraceptive use. A pooled analysis of studies conducted through the early 1990s found that only current or recent users were at higher risk of breast cancer, but two more recent studies provide conflicting results with one showing no impact of oral contraceptives on risk of any group of women and the other showing an increase in risk with use. Among women participating in observational studies who are current or recent users of oestrogen therapy and who have relatively long durations of use, risk of breast cancer is modestly elevated; results from the Women's Health Initiative trial, based on a relatively short duration of oestrogen use are consistent with these observations. Oestrogen therapy increases endometrial cancer risk dramatically. It may also increase the risk of ovarian cancer; however more study of this issue is needed as the literature is inconsistent. Combined oestrogen and progestin therapy increases breast cancer risk by as much as 10% per year of use. Endometrial cancer risk is not elevated when combined therapy is given in a cyclic manner with progestin administered only part of the time and it is reduced when both oestrogen and progestin are administered on a daily basis. As with oestrogen therapy, the impact of combined therapy on ovarian cancer risk is uncertain.