Observational epidemiologic studies and randomized trials have reported a protective effect of oral hormonal replacement therapy on risk of colorectal cancer. Only one previous prospective study, the Women's Health Initiative Observational Study, has reported on the relationship between endogenous hormones and incident colorectal cancer. Contrary to expectation, the investigators found that women with higher circulating estradiol levels were at increased risk of developing colorectal cancer. We conducted a case-control study nested within the New York University Women's Health Study prospective cohort to evaluate the association between endogenous levels of estrone, estradiol, and sex hormone-binding globulin (SHBG) with risk of colorectal cancer. We measured hormones and SHBG in serum samples collected at enrollment from a total of 148 women who subsequently developed colorectal cancer and 293 matched controls. Circulating estrone levels were positively associated with risk of colorectal cancer: The odds ratio for the highest versus lowest quartile of estrone was 1.8 (95% confidence interval, 1.0-3.3). We found a nonsignificant inverse association between SHBG and colorectal cancer, which disappeared after adjusting for body mass index. We did not find an association between estradiol and colorectal cancer risk, but we cannot rule out a potential association because of substantial laboratory error in the measurement. Our results suggest that endogenous estrone is associated with increased risk of colorectal cancer in postmenopausal women.