Objectives: The main objective is to characterize the fecal microbiota and its association with levels of systemic estrogens among randomly sampled postmenopausal female members of a health maintenance organization (HMO). A secondary objective will determine the proportions of women who consent and then provide questionnaire data, a fecal specimen to characterize the microbiota, and a urine specimen to quantify systemic estrogens; and differences between participants (N=60) and non-participants. A third objective will determine whether consecutive, newly diagnosed postmenopausal breast cancer cases have similar participation rates, fecal microbiota characteristics and urine estrogen levels compared to the randomly sampled women.