Aromatase inhibitors (AIs) reduce recurrences and mortality in postmenopausal patients with oestrogen receptor positive (ER+) breast cancer (BC), but >20% of patients will eventually relapse. Given the limited understanding of intrinsic resistance in these tumours, here we conduct a large-scale molecular analysis to identify features that impact on the response of ER + HER2- BC to AI. We compare the 15% of poorest responders (PRs,n= 177) as measured by proportional Ki67 changes after 2 weeks of neoadjuvant AI to good responders (GRs,n= 190) selected from the top 50% responders in the POETIC trial and matched for baseline Ki67 categories. In this work, lowESR1levels are associated with poor response, high proliferation, high expression of growth factor pathways and non-luminal subtypes. PRs having highESR1expression have similar proportions of luminal subtypes to GRs but lower plasma estradiol levels, lower expression of estrogen response genes, higher levels of tumor infiltrating lymphocytes and immune markers, and moreTP53mutations.