Background/Significance: Commensal microbes (the microbiota), particularly in the gut, are required for human health and are postulated to affect cancer risk through several mechanisms. This proposed study builds upon a pilot within the NCI Division of Cancer Epidemiology and Genetics (DCEG) that identified highly acceptable methods for collecting fecal specimens and significant correlation between fecal microbial beta-glucuronidase activity and a marker of breast cancer risk, urine levels of estrogens. The proposed study will determine the correlation between levels of fecal enzyme activities and systemic estrogens in a random sample of postmenopausal women at Kaiser Permanente Colorado (KPCO). Demonstration of an association between the fecal microbiota and systemic estrogens would help to motivate future studies of how microbes affect cancer risk.