Background: Aromatase inhibitors (AIs) are adjuvant therapies (tx) of choice for post-menopausal estrogen-receptor (ER) positive (+ve) breast cancer (BC) because of superior efficacy and favorable side-effects over selective ER modulators such as tamoxifen. However, tamoxifen is thought to have a better bone safety profile as it binds specifically to ER +ve cells, whereas AIs block the conversion of androgens to estrogens. AIs are associated with increased risk of bone loss and osteoporosis compared to tamoxifen. In the US, many women are diagnosed with ER +ve BC and, since 2000, adjuvant AI use increased in tandem with a decrease in tamoxifen. As higher rates of utilization are anticipated, we report estimates of the prevalence (1-year period prevalence) of women with non-metastatic BC treated with AIs. Methods: The Oncology Services Comprehensive Electronic Records (OSCER) database was used to identify women with BC (ICD-9 174), ≥ 1 clinic visit, and confirmed AI (anastrozole, letrozole, or exemestane) in 2011. Women with metastases (ICD-9 196-198, stage IV, or M1 disease) were excluded. OSCER captures electronic medical record data on >500,000 cancer patients since 2004 from US outpatient community and hospital affiliated oncology practices. OSCER is projected nationally through methods of direct estimations utilizing claims data (Smith et al. submitted). To identify subgroups of women at increased risk of bone loss, estimates were stratified by age and number of AIs received. Bone loss was defined by diagnosis of osteoporosis (ICD-9 733.0), osteopenia (ICD-9 733.90), or receipt of bone tx either before initiation of first AI (proxy for pre-existing bone loss) or any time before or during 2011 (proxy for current bone loss). Results: When projected to the US population, it is estimated that 489,664 (95% CI 467,642-511,686) women with non-metastatic BC were treated with an AI in 2011. Of these, 52% (253,672 [95% CI 239,229-268,115]) were 65 or older. The table summarizes results for the projected cohorts. These data indicate that 19% of women receiving AIs were in the pre-/peri-/early post-menopausal age group (55 or younger), and approximately 66% of women treated with AIs did not have evidence of pre-existing bone loss (i.e., either osteoporosis diagnosis or receipt of bone tx before AI). Conclusions: This is the first study to estimate prevalence of women with non-metastatic BC treated with AIs in the US. As a large proportion of women are diagnosed with ER+ve BC and eligible for AIs (Jemal et al. 2007), a growing number will face increased subsequent risk of fractures. These findings suggest that up to 500,000 women annually are treated with AIs, and over 40% have evidence of bone loss before or during AI. These women will face increased risk of fractures, highlighting the need for prevention of fractures and intervention with targeted bone tx to increase bone mass and prevent or treat osteoporosis. [ABSTRACT FROM AUTHOR]