Research Question: Do Aromatase Inhibitors Decrease Intestinal Calcium Absorption? Study Design: Postmenopausal women with early stage breast cancer initiating aromatase inhibitor adjuvant therapy will participate in this two-month study. The primary study outcome is the change in intestinal calcium absorption following such therapy. Secondary outcomes are the changes in bone resorption markers and musculoskeletal signs and symptoms after initiation of therapy. We will interview women and review their medical records to determine eligibility. Eligible subjects will undergo two calcium absorption studies. The first study will determine their baseline calcium absorption, and the second study will occur after taking an aromatase inhibitor daily for at least 6 weeks. Women will present to the research unit in the early morning and receive an oral and intravenous stable calcium tracer with breakfast. Over the next 24 hours, we will collect all urine for measurement of its calcium content. During each inpatient stay, we will assess musculoskeletal symptoms by questionnaire and joint examination. Each woman will complete a four-day diet diary twice during the study.