The incidence of breast cancer in women varies with age, mammary gland mass and exposure to endogenous and exogenous hormones. Age is the single most important factor and if, as projected, 32% of women will be aged >60 years by 2050, world breast cancer incidence will exceed the current 10(6) per year. Hormonal influences that affect growth of the mammary gland increase the risk of breast cancer; for example earlier menarche and later menopause. Childbearing protects against later development of breast cancer, and breastfeeding further decreases the risk. The breast cancer risk declines more with increasing total duration of breastfeeding. Exposure to hormonal contraceptives has been evaluated in a combined reanalysis of data from 51 epidemiological studies. There is a small transient increase in the relative risk of breast cancer among users of oral contraceptives but, since use typically occurs at young ages when breast cancer is relatively rare, such an increase would have little effect on overall incidence rates. In contrast, exposure to menopause hormone treatment occurs when the baseline risk of breast cancer is higher, and epidemiological studies and randomized controlled trials consistently find an increase in breast cancer risk with exposure to combined estrogen and progestogen. Women with a family history of breast cancer in first degree relatives have an increased risk of breast cancer but there is no evidence to suggest that this differs according to a woman's use of oral contraceptives or menopause hormone treatment. Selective estrogen receptor modulators are useful in the treatment and/or prevention of breast cancer depending on the specific agonist or antagonist effects on estrogen target tissues.