The objectives of this study were to examine: (1) changes in bone formation (osteocalcin) and bone resorption (cross-linked N-telopeptides of bone type I collagen [NTXs]) markers, as well as calcium, phosphorus, and intact parathyroid hormone, over the first 6 months of aromatase inhibitor (AI) therapy among a cohort of breast cancer patients compared with a group of unexposed women without a history of cancer; and (2) whether bone marker changes were associated with musculoskeletal pain. Eligible breast cancer patients (n = 49) and postmenopausal women without a history of cancer (n = 117) were recruited and followed for 6 months. At baseline, 3 months, and 6 months, a questionnaire was administered to assess pain and medication use, and a blood sample was drawn. Results showed that, among the breast cancer patients, calcium concentrations decreased significantly (−7.8% change; p = 0.013) and concentrations of NTXs increased significantly from baseline to 6 months (9.6% change; p = 0.012). Changes were not observed for women in the comparison group. Statistically significant differences in percent change between the breast cancer patients and the women in the comparison group were observed for calcium at 6 months (−7.8% versus 0.0%; p = 0.025), phosphorus at 6 months (−5.1% versus 16.7%; p = 0.003), NTXs at 6 months (9.6% versus −0.7%; p = 0.017), and osteocalcin at 6 months (11.5% versus −3.6%; p = 0.016). No statistically significant associations were observed between bone turnover marker changes and musculoskeletal pain among the breast cancer patients, although baseline NTXs were higher among women with onset or increase in pain compared with those reporting no pain (p = 0.08). Findings from this study suggest that AIs cause changes in bone turnover during the first 6 months of treatment; however, these changes are not associated with musculoskeletal pain. Breast cancer patients initiating AI therapy should be assessed and monitored for fracture risk using known clinical risk factors, including bone density, and managed appropriately. © 2012 American Society for Bone and Mineral Research.