Background: Aromatase inhibitors (AIs) compose the backbone of adjuvant endocrine therapy for early-stage HR+ breast cancer. Prior studies have demonstrated equivalence in efficacy between aromatase inhibitors in current clinical use. Thus choice of aromatase inhibitors is initially influenced by provider patterns and tailored based on adverse side effects. This study retrospectively evaluates the prevalence of intolerance to AIs warranting a change in therapy. Methods: We identified 181 post-menopausal patients with early-stage, HR+ breast cancer within a database of women who underwent oncotype testing at a large, urban medical center, with 167 (92%) receiving an AI. Patients were excluded if they had received chemotherapy. The Kaplan-Meier method was used to estimate median duration of endocrine therapy (ET). The Fisher’s exact test was used to compare proportions of patients requiring a switch in adjuvant AI and the Wilcoxon rank-sum test was used to compare age distributions. Results: Of the patients identified, 153 (90%) had stage IA disease with a median oncotype RS of 19. 167 (92%) patients received an AI as adjuvant therapy. Median duration of endocrine therapy (ET) was 83.5 months, with over 90% of patients on ET more than 5 years. Among the 152 patients with available data on ET interruptions, 52 (34%) patients had a change or interruption in their ET of these 40 (77%) were attributed to a medication side effect. The AI prescribed most frequently as initial therapy was anastrozole 138/165 (84%). 33 (24%) of patients who received anastrozole required a drug switch within the class, (no sig difference based on initial adjuvant therapy; p=0.4281). 12 (7%) patients required a switch from an AI to tamoxifen at some point during therapy. Changes in AI adjuvant therapy were most commonly attributed to joint pains 24 (46%), followed by hot flashes 2 (4%) and weight gain, 2 (4%). There was no statistically significant difference in median age between patients who interrupted ET due to medication side effect or other reason; p=0.2906. Conclusions: This study enforces previous findings that a significant proportion of women require a change in their adjuvant ET, most commonly due to adverse medication effects such as joint pain. The majority of the women in this database were initiated on anastrozole as initial adjuvant ET of whom 24% required a drug switch.