To test the hypothesis that high levels of endogenous oestrogens increase the risk for developing breast cancer, concentrations of oestrone, oestradiol and oestriol were measured in 24 h urine samples from 1000 women participants in a prospective study of breast cancer on the island of Guernsey. Sixty-nine subjects were diagnosed with breast cancer subsequent to urine collection. Among women who were premenopausal at the time of urine collection, cases excreted less oestrogen than controls; the odds ratios (95% CI) for breast cancer in the middle and upper thirds of the distribution of oestrogen excretion, in comparison with the lower third (reference group, assigned odds ratio = 1. 0), were 0. 5(0. 2-1. 2) and 0. 4(0. 2-1. 1) respectively for oestrone, 0. 8(0. 4-1. 8 and 0. 4(0. 2-1. 1) for oestradiol, 0. 7(0. 3-1. 6) and 0. 7(0. 3-1. 6) for oestriol and 0. 9(0. 4-2. 0) and 0. 5(0. 2-1. 3) for total oestrogens. Among women who were post-menopausal at the time of urine collection, the trend was in the opposite direction, with an increase in risk associated with increased oestrogen excretion; the odds ratios were 0 VSports手机版. 9(0. 3-2. 2) and 1. 1(0. 5-2. 8) for oestrone, 0. 8(0. 3-2. 3) and 1. 9(0. 8-4. 6) for oestradiol, 1. 5(0. 6-3. 9) and 1. 8(0. 7-4. 6) for oestriol and 0. 9(0. 4-2. 6) and 1. 9(0. 7-4. 7) for total oestrogens. The trends of increasing risk with increasing oestrogen excretion among post-menopausal women were statistically significant for oestradiol (P = 0. 022) and for total oestrogens (P = 0. 016). We conclude that high levels of endogenous oestrogens in post-menopausal women are associated with increased breast cancer risk, but that the relationship of oestrogens in premenopausal women with risk is unclear. .