Background: Obesity is an adverse prognostic factor in BC. Mixed results are reported for the relative efficacy of AIs compared to tamoxifen (T) in obese ER+ BC patients. Our purpose was to conduct a meta-analysis of adjuvant randomised trials of AIs vs T assessing the impact of body mass index (BMI). Methods: We identified four studies evaluating BMI and endocrine therapy. Of these, 3 were randomised (non-steroidal AIs vs T) and were evaluable for the aggregation of results for DFS and OS in our meta-analysis. We extracted published data from ATAC, ABCSG-12 and BIG01-98, analyzed according to standard meta-analytic techniques. Results: A total of 11,383 patients were included in our study. BMI>25 is associated with reduced disease free survival (DFS) and a trend towards worse overall survival (OS) (Table 1). A significantly shorter DFS was seen for patients with BMI>25 treated with an AI while a trend was seen for OS. Reduced relative efficacy was seen for DFS for AIs compared to T for BMI<25 (HR=0.78; 95%CI 0.66- 0.91; p=0.002) and a trend for BMI>25 (HR=0.85; 95%CI 0.70- 1.02; p=0.08). The test for interaction was not significant (p=0.48), with similar results for OS for BMI<25 (HR=0.79; 95%CI 0.63-0.9; p=0.009) and BMI>25 (HR=0.98; 95%CI 0.61-1.60; p=0.95). The test for interaction was not significant (p=0.37). Notably, significant heterogeneity in patients treated with anastrozole and a BMI>25 did not allow a comparison between anastrozole and letrozole. Conclusions: BMI>25 has a negative prognostic effect in BC. AIs demonstrate improved outcomes in normal weight BC patients (BMI<25). Obesity was associated with observed relative reduced efficacy of AIs; however, we were not able to detect a significant interaction between BMI and treatment effect. Further analyses into the differing impact of type of AIs on BC outcomes in obese patients are warranted.