Abstract Systemic estrogen level strongly determines risk for postmenopausal breast cancer (BC). We hypothesize that differences in hormone metabolism by the gut microbiota may contribute to BC risk by modulating enterohepatic circulation of estrogens. We conducted a study to determine the feasibility of enrolling healthy postmenopausal members of Kaiser Permanente Colorado (KPCO) in a fecal microbiome study. With electronic medical records, we created a pool of potentially eligible women aged 55-69 who recently had a negative screening mammogram. We excluded those with any hormone prescription in the prior 12 months, an antibiotic prescription in the prior 6 months, or history of cancer, inflammatory bowel disease or other conditions likely to affect fecal microbiota. A packet (invitation letter, informed consent form, information pamphlet, and opt-out postcard) was sent to 250 randomly sampled women. Women who have not returned the postcard are contacted by KPCO research staff to determine interest in participating, verify eligibility, and obtain verbal informed consent. Consented participants were mailed a second packet with a self-administered risk factor questionnaire, link to an on-line diet questionnaire, collection kit for urine (to measure 15 estrogens) and stool specimens, and shipping instructions to the laboratory. Enrollees have spent an estimated 1-2 hours completing all recruitment and data collection components. To date, 60 (24%) have enrolled, 70 (28%) refused (most via the opt-out card), and the remainder are pending phone contact. As shown in the table, the enrolled women are similar to the total sample; slightly more of them are non-Hispanic white, and they are more likely to have a history of breast biopsy. These data demonstrate that we can successfully recruit an apparently representative sample of women into a microbiome study. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5489. doi:1538-7445.AM2012-5489