Study summary: The current study was designed to determine the associations between alcohol consumption and 11 hormones and peptides in postmenopausal breast cancer survivors and to evaluate whether these relationships were different for those who take tamoxifen. A food questionnaire was used to assess alcohol intake 30 months after breast cancer diagnosis in 490 postmenopausal women from three western U.S. states. In addition, a fasting blood sample was analyzed to determined circulating levels of estrone, estradiol, free estradiol, C-peptide, testosterone, free testosterone, dehydroepiandrosterone sulfate (DHEAS), leptin, sex hormone-binding globulin, insulin-like growth factor-I (IGF-I), and IGF-binding protein-3. The associations between alcohol intake and serum hormone and peptide levels were found to differ by tamoxifen use; there were statistically significant inverse associations between alcohol intake and both leptin and sex hormone-binding globulin values, but only among tamoxifen users. In women who were not using tamoxifen (but not those on tamoxifen), the study found a positive association between alcohol intake and DHEAS. The associations found for DHEAS and sex hormone-binding globulin values are in a direction considered unfavorable for breast cancer prognosis. The authors conclude that tamoxifen may modify the associations between alcohol intake and serum hormones and peptides in a manner that increases risk for postmenopausal breast cancer survivors.