Non-contraceptive exogenous oestrogen therapy and breast cancer.

Previous evidence of an increased risk of breast cancer in association with the use of non-contraceptive exogenous oestrogen therapy has not been consistent. In this population-based case-control study of breast cancer (comprising 451 case-control pairs in all) the relationship between the use of non-contraceptive exogenous oestrogen therapy and the risk of breast cancer was examined in the 281 case subjects and 288 control subjects (262 case-control pairs) who were postmenopausal, since the major reported indication for the use of exogenous oestrogen therapy is for the relief of menopausal symptoms. "Ever use" of exogenous oestrogen therapy was associated with an adjusted relative risk of 1.03 (95% confidence interval, 0.62-1.69). The risk of breast cancer was not associated with the duration of use, years since first use, or years since last use of exogenous oestrogen therapy. Stratified analyses of breast cancer showed a 35% increase in risk in nulliparous women that is associated with the use of exogenous oestrogen therapy, a 40% reduction in risk in women with a history of benign breast disease (both statistically not significant), and a statistically significant 70% reduction in risk in women with a history of bilateral oophorectomy. The results of this study should be weighed against those of several earlier studies which suggest caution in the prescription of replacement oestrogen therapy for those women who are known already to be at an increased risk of breast cancer.