Background: Chemotherapy efficacy in early-stage hormone receptor-positive (HR+) breast cancer (BC) according to menopausal status needs a biological explanation., Methods: We compared early-stage HR+ BC biological features before and after (neo)adjuvant chemotherapy or endocrine therapy (ET), and assessed oestrogen receptor (ER) pathway activity in both pre- and post-menopausal patients. The nCounter platform was used to detect gene expression levels., Findings: In 106 post-menopausal patients with HR+/HER2-negative BC randomized to neoadjuvant chemotherapy or ET (letrozole+ribociclib), a total of 19 oestrogen-regulated genes, including progesterone receptor (PGR), were found downregulated in the ET-based arm-only. We confirmed this finding in an independent dataset of 20 letrozole-treated post-menopausal patients and found, conversely, an up-regulation of the same signature in HR+/HER2-negative MCF7 cell line treated with estradiol. PGR was found down-regulated by 2 weeks of ET+anti-HER2 therapy in pre-/post-menopausal patients with HR+/HER2-positive (HER2+) BC, while anti-HER2 therapy alone increased PGR expression in HR-negative/HER2+ BC. In 88 pre- and post-menopausal patients with newly diagnosed HR+/HER2-negative BC treated with chemotherapy, the 19 oestrogen-regulated genes were found significantly downregulated only in pre-menopausal patients. In progesterone receptor (PR)+/HER2-negative BC treated with neoadjuvant chemotherapy (n=40), tumours became PR-negative in 69.2% of pre-menopausal patients and 14.8% of post-menopausal patients (p=0.001). Finally, a mean decrease in PGR levels was only observed in pre-menopausal patients undergoing anti-HER2-based multi-agent chemotherapy., Interpretation: Chemotherapy reduces the expression of ER-regulated genes in pre-menopausal women suffering from hormone-dependent BC by supressing ovarian function. Further studies should test the value of chemotherapy in this patient population when ovarian function is suppressed by other methods., Funding: Instituto de Salud Carlos III, Breast Cancer Now, the Breast Cancer Research Foundation, the American Association for Cancer Research, Fundació La Marató TV3, the European Union's Horizon 2020 Research and Innovation Programme, Pas a Pas, Save the Mama, Fundación Científica Asociación Española Contra el Cáncer, PhD4MDgrant of "Departament de Salut", exp SLT008/18/00122, Fundación SEOM and ESMO. Any views, opinions, findings, conclusions, or recommendations expressed in this material are those solely of the author(s)., Competing Interests: Declaration of Competing Interest Dr. Prat reports grants and personal fees from Pfizer, grants and personal fees from Lilly, personal fees from Nanostring tecnologies, grants and personal fees from Amgen, grants from Roche, personal fees from Oncolytics Biotech, personal fees from Daiichi Sankyo, personal fees from PUMA, personal fees from BMS, from Daiichi Sankyo, outside the submitted work. Dr. Perou reports personal fees from Bioclassifier LLC, outside the submitted work; in addition, Dr. Perou has a U.S. Patent No. 12,995,459 with royalties paid. Dr. Saura reports personal fees from AstraZeneca, Daiichi Sankyo, Eisai, Exact Sciences, Exeter Pharma, F. Hoffmann - La Roche Ltd, MediTech, Merck Sharp & Dohme, Novartis, Pfizer, Philips, Piere Fabre, Puma, Roche Farma, Sanofi-Aventis, SeaGen, and Zymeworks outside the submitted work. Dr. Ciruelos reports personal fees from Roche, Pfizer, Lilly, Novartis, Astra-Zeneca/Daiichy Sankio and from MSD outside the submitted work. Dr. Gavilá reports grants from Novartis, Pfizer, Astra-Zeneca and Lilly, outside the submitted work. Dr. Soberino reports grants and personal fees from MSD, grants from Sanofi and personal fees from Roche outside the submitted work. Dr. Carey reports uncompensed relationship with Sanofi, G1 Therapeutics, Genentech/Roche, AstraZeneca/Daiichi Sankyo, Apitude Health and Exact Sciences; in addition Dr. Carey declared that Companies who have provided research funds to her institution in the past 1–2 years were Syndax, Immunomedics, Novartis, Nanostring technologies, Abbvie, Seattle Genetics and Veracyte, outside the submitted work. Dr Bellett declares advisory board participation for Lilly, Pfizer and Novartis, as well as travel expenses from Pfizer, outside the submitted work. Dr. Muñoz reports expert testimony for Eisai, Novartis and Roche, advisory board for Pierre Fabre and travel grants from Lilly, Pfizer and Roche, outside the submitted work. Dr. Oliveira reports grants from Pfizer, grants, personal fees and non-financial support from Roche, grants and personal fees from Genentech, grants, personal fees and non-financial support from Novartis, grants and personal fees from Seattle Genetics, grants and personal fees from Astra-Zeneca, grants and personal fees from PUMA Biotechnolgy, grants from Immunomedics, grants from Boehringer-Ingelheim, non-financial support from Eisai, Grunenthal, GP Pharma and Pierre Fabre, outside the submitted work. Dr. Pernas reports personal fees and non-financial support from Novartis, personal fees from AstraZeneca, Daiichi-Sankyo, Polyphor, Seattle Genetics, Eisai, Roche and Pierre-Fabre, outside the submitted work. Dr. Cortes reports personal fees and travel expenses from Pfizer, personal fees from Lilly, Servier, Athenex, personal fees and travel expenses and advisory/honoraria from Roche, personal fees from Polyphor, personal fees and travel expenses from Daiichi Sankyo, Novartis and Eisai, personal fees from MSD, GSK, Astra-Zeneca, Celgene, Cellestia, Biothera, Merus, Seattle Genetics, Erytech, Leuko, Bioasis, Clovis Oncology, Boerhinger-Ingelheim, Samsung Bioepis, and stocks from MedSIR, outside the submitted work. The other authors have nothing to declare., (Copyright © 2021 The Author(s). Published by Elsevier B.V. All rights reserved.)