Multiple lines of evidence support a central role of hormones in the etiology of breast, endometrial and ovarian cancers. Evidence of an association between circulating hormones and these cancers varies by both hormone and cancer site, with the most consistent associations observed for sex steroid hormones and breast cancer risk among postmenopausal women. Recently, evidence has begun to accumulate suggesting an important role for endogenous hormones in premenopausal breast cancer, endometrial cancer and possibly ovarian cancer. In this chapter, prospective epidemiologic studies, where endogenous hormones are measured in study subjects prior to disease diagnosis, are summarized. Overall, a strong positive association between breast cancer risk and circulating levels of both estrogens and testosterone has now been well confirmed among postmenopausal women; women with hormone levels in the top 20% of the distribution (versus bottom 20%) have a two-to-three-fold higher risk of breast cancer. Evidence amongpremenopausal women is more limited, though increased risk associated with higher levels of testosterone is consistent. Evidence to date of hormonal associations for endometrial cancer is limited, though a strong association with sex steroid hormones is suggested. Studies of ovarian cancer have been few and small with no consistent associations observed with endogenous hormones. Clearly more evaluation is needed to confirm the role of endogenous hormones in premenopausal breast cancer, endometrial cancer and ovarian cancer.