Endogenous Sex Hormones and Breast Cancer in Postmenopausal Women: Reanalysis of Nine Prospective Studies The Endogenous Hormones and Breast Cancer Collaborative Group

volved in the etiology of breast cancer, but there are only a few prospective studies on endogenous sex hormone levels and breast cancer risk. We reanalyzed the worldwide data from prospective studies to examine the relationship be-tween the levels of endogenous sex hormones and breast cancer risk in postmenopausal women. Methods: We ana-lyzed the individual data from nine prospective studies on 663 women who developed breast cancer and 1765 women who did not. None of the women was taking exogenous sex hormones when their blood was collected to determine hor-mone levels. The relative risks (RRs) for breast cancer as-sociated with increasing hormone concentrations were esti-mated by conditional logistic regression on case–control sets matched within each study. Linear trends and heterogeneity of RRs were assessed by two-sided tests or chi-square tests