Multiple lines of evidence support a central role of hormones in the etiology of breast cancer. In epidemiologic studies, considerable effort has focused on delineating the role of endogenous hormones in risk of breast cancer among postmenopausal women. Recently, substantial additional data has accrued from prospective studies where endogenous hormones are measured in study subjects prior to disease diagnosis. In this review, the epidemiologic evidence linking sex steroids, prolactin and insulin-like growth factors (IGF) with subsequent risk of breast cancer in postmenopausal women is summarized and evaluated. Overall, a strong positive association between breast cancer risk and circulating levels of both estrogens and androgens has now been well confirmed; women with hormone levels in the top 20% of the distribution (versus bottom 20% have a 2- to 3-fold higher risk of breast cancer. Accumulating data also indicate a significant positive association with prolactin levels, although additional confirmation is needed. In contrast, no important link has been found between circulating levels of IGF-I (and its binding protein, IGFBP-3) and breast cancer risk in postmenopausal women.