Background: Hormone receptor positive (HR+) breast cancer comprises the largest subgroup of breast cancer. Aromatase Inhibitors (AI) are a key treatment for HR+ BC patients (pts) and reduce mortality. Aromatase Inhibitor-Associated Musculoskeletal Symptoms (AIMSS), defined as myalgias, arthralgias, or joint stiffness, occur in up to 50% of pts leading to low adherence to and often discontinuation of therapy. Little is known of the mechanism of AIMSS or its predisposing risk factors. This study aims to identify risk factors associated with AIMSS development in BC patients on AI therapy. Methods: We conducted a medical record review of pts with non-metastatic HR+ BC on adjuvant AI therapy between January 2009 and June 2017 at the University of Maryland Comprehensive Cancer Center. This study included 194 ptswho were free of arthralgia prior to AI therapy. We analyzed pts’ demographics, lifestyle factors, reproductive history, tumor characteristics, medications, cancer treatment, co-morbidities, AI type, onset and severity of AIMSS. Severe AIMSS was defined as requiring change in AI therapy or discontinuation. Multivariable-adjusted logistic regression was used to identify risk factors for severe AIMSS. Results: The mean age of participants was 61. The mean BMI at diagnosis was 30 kg/m². 41% of pts were White, 40% were Black, 7% other and 12% unknown. Most (79%) did not have a history of tamoxifen and 16% were on GnRH agonists. Most (71%) used letrozole as initial AI therapy; 18% anastrozole; and 11% exemestane. 56% experienced AIMSS while on AI therapy and 20% required change or hold of AI therapy. 4% permanently discontinued AI due to AIMSS severity. BMI at diagnosis was significantly positively associated with risk of AIMSS. Multivariate odds ratio (95% confidence intervals) comparing the highest to lowest tertile of body mass index (BMI) at diagnosis was 4.01 (1.07-10.90; P_trend: 0.05). There were no significant associations with race, smoking, reproductive factors, type of AI therapy, tamoxifen use prior to AI therapy, medication use, experience of other cancer treatments, and tumor characteristics. Conclusions: 56% of BC pts on adjuvant AI therapy experienced AIMSS. 24% of these changed, held or discontinued AI regimen due to severe AIMSS. Higher BMI at diagnosis was associated with a higher risk of AIMSS. Our results confirm clinical significance of AIMSS among BC pts on AI therapy and suggest BMI as a modifiable factor for AIMSS. A larger study is warranted to replicate our findings and seek other possible risk factors for AIMSS.