Serum oestradiol and breast cancer risk.

Breast cancer risk is increased by early menarche and late menopause, suggesting that the long duration of exposure of the breasts to the high levels of ovarian steroids in premenopausal women increases risk. Recent prospective studies have shown that postmenopausal women who develop breast cancer have significantly greater prediagnostic serum concentrations of oestradiol than postmenopausal women who remain healthy. Estimation of long-term oestradiol concentrations in premenopausal women is difficult, and few data are available from prospective studies, but these are compatible with the hypothesis that relatively high oestradiol concentrations in premenopausal women are also associated with an increase in breast cancer risk. Women in populations with low breast cancer rates have low serum oestradiol concentrations both before and after the menopause. The serum concentration of oestradiol is probably a major determinant of breast cancer risk, but more data are needed to confirm this and to investigate the possible roles of other sex hormones.