NEOADYUVANT ENDOCRINE THERAPY IN BREAST CANCER. Neoadjuvant chemotherapy is an accepted strategy for patients with locally advanced breast cancer. This approach increases the possibilities of conservative treatment and improves the resectability rates of initially unresectable tumors. In addition, preoperative systemic therapy allows the evaluation of prognostic and predictive factors, dynamically and in vivo. Since over 80% of these tumors express estrogen receptors (ER), endocrine therapy seems a logical treatment to employ in the neoadjuvant setting. The advent of new drugs that regulate the ER function, along with the results of several clinical studies with the use of neoadjuvant endocrine therapy, support the feasibility and safety of utilizing this strategy before surgery. We herein analyze the available clinical evidence about the use of neoadjuvant therapy aiming to regulate the activity of the ER. We also discuss the value of predictive factors that could help the oncologist to select those patients most likely to benefit from this approach and the role of endocrine therapy as a research instrument.