Background Whereas the genomic landscape of endocrine‐resistant breast cancer has been intensely characterized in previously treated cases with local or distant recurrence, comparably little is known about genomic alterations conveying primary non‐responsiveness to endocrine treatment in luminal early breast cancer. Methods In this study, 622 estrogen receptor‐expressing breast cancer cases treated with short‐term preoperative endocrine therapy (pET) from the WSG‐ADAPT trial (NCT01779206) were analyzed for genetic alterations associated with impaired endocrine proliferative response (EPR) to 3‐week pET with tamoxifen or aromatase inhibitors. EPR was categorized as optimal (post‐pET Ki67
Early recognition of endocrine resistance could be enabled by impaired suppression of proliferation after short‐term preoperative endocrine therapy (pET). Impaired endocrine proliferative response (EPR, post‐pET Ki67 ≥10%) sorts out cancers carrying TP53 mutations.