Early stage hormone receptor positive breast cancer is typically treated with adjuvant endocrine therapy in order to decrease risk of breast cancer recurrence and to improve overall survival from the disease. Typical agents used for treatment include tamoxifen and the aromatase inhibitors. In postmenopausal women, aromatase inhibitor therapy is increasingly common because it is associated with fewer long-term serious toxicities compared to tamoxifen. However, aromatase inhibitors cause arthralgias in 40-50% of patients, which can influence adherence to therapy and can lead to treatment discontinuation in a minority of cases. The mechanism underlying development of this toxicity remains unclear, and predictors of who will develop these symptoms remain undefined. This observational study is designed to collect patient-reported outcomes and serial serum samples in order to investigate potential etiologies of this bothersome toxicity.

Inclusion Criteria:

• Stage I-III breast cancer who are scheduled to receive endocrine therapy with an aromatase inhibitor
• All prior surgery, chemotherapy, and radiation therapy should be complete or should be completed by the time of AI treatment initiation (within 28 days of study enrollment)
• Age 21 and above and postmenopausal

Postmenopausal women with hormone receptor positive breast cancer who are initiating therapy with an aromatase inhibitor