Abstract Introduction: Breast cancer is the most common cancer in women worldwide, and 70% of breast cancer deaths occur in women from low-income and middle-income countries. In Brazil there were 14388 deaths due to this disease in 2013 and an estimate of over 58000 new cases in 2016. Neoadjuvant endocrine therapy (NET) is an attractive alternative to Neoadjuvant chemotherapy (NAC) for Hormone Receptor-positive tumors and could be a resources-saving strategy of treatment. Methods: We built a decision analysis model of breast cancer treatment to compare a NET schema, with response based on the evaluation of Ki-67, against the surgery followed by adjuvant chemotherapy (AC) and radiation therapy (RT) standard-of-care as two competing approaches to breast cancer management. Our objective is to determine whether tailoring chemotherapy treatment based on response to neoadjuvant endocrine therapy is a cost-effective approach. The NET schema is based on the ACOSOG Z1031B trial, in which post-menopausal women with estrogen receptor-positive, HER-2 negative disease would receive 4 weeks of NET followed by a core-needle biopsy for Ki-67 evaluation. If Ki-67 were lower than 10%, patients would continue in NET for 16-18 weeks followed by surgery and RT according to international guidelines. The indication of AC in these patients would be based on the preoperative endocrine prognostic index (PEPI). Patients with a PEPI score equal to zero would be spared from AC. If Ki-67>10%, patients would be triaged to NAC or surgery. The cost-effectiveness analysis was conducted using a Markov model from the provider's perspective, in this case the Brazilian Health ministry. Healthcare costs, in the form of charges from the hospitals to the health ministry, were obtained from cost tables available at the federal government's webpage. In the Markov model, possible health states were disease-free, local relapse, metastatic disease and death.Transition probabilities and mortality rates were extracted from randomized studies. Our assumptions were that both treatment strategies have similar clinical outcomes and that Ki-67 is a reliable method to triage patients to NAC or surgery. We performed one-way sensitivity analysis to assess the impact of the failure of the Ki-67 test on cost-effectiveness. Results: Our model shows that the NET schema dominates the standard-of-care strategy. Costs were R$ 47799.89 per patient for the NET strategy and R$79809.24 for the standard-of-care strategy. There was an incremental cost saving of R$32009.36 per patient for the NET strategy compared to the standard-of-care strategy. Cost-effectiveness of the NET strategy was R$2612.63 and R$4369.11 for the standard-of-care. Considering the willingness-to-pay of R$ 85494.00, defined by the World Health Organization as three times the gross domestic product per capita, the standard-of-care strategy would only be more cost-effective in the scenario of a Ki-67 test that misclassifies patients more than 9.1% of the time. Conclusion: The use of response to neoadjuvant endocrine treatment based on Ki-67 analysis as a way to tailor locally advanced breast cancer treatment is a cost-saving strategy in the presence of robust biomarkers. Citation Format: Goncalves R, Reinert T, Ellis MJ, Sarian LO, Filassi JR. Cost-effectiveness analysis of locally advanced estrogen receptor-positive, HER-2 negative breast cancer care using a tailored treatment approach in Brazil [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P4-12-04.