Purpose: To investigate the effects of body mass index (BMI) on the response to neoadjuvant chemotherapy (NACT) in Turkish patients with local and locally advanced breast cancer.Methods: The pathological responses for the breast and axilla were assessed according to the Miller-Payne grading (MPG) system. Tumors were grouped into molecular phenotypes and classified as response rates according to the MPG system after completion of NACT. A 90% or greater reduction in tumor cellularity was considered as good response to treatment. Additionally, patients were grouped according to BMI into <25 (group A) and >= 25 (group B).Results: In total, 647 Turkish women with breast cancer were included in the study. In the univariate analysis, age, menopause status, tumor diameter, stage, histological grade, Ki-67, estrogen receptor (ER) status, progesterone receptor (PR) status, human epidermal growth factor receptor 2 (HER2) status, and BMI were assessed to determine which of these factors were associated with a >= 90% response rate. Stage, HER2 positivity, triple-negative breast cancer (TNBC; ER-negative, PR-negative, and HER2 negative breast cancer), grade, Ki67 levels, and BMI were found to be the statistically significant factors for a >= 90% response rate. In the multivariate analysis, grade III disease, HER2 positivity, and TNBC were found to be the factors associated with a high pathological response. Meanwhile, hormone receptor (HR) positivity and a higher BMI were associated with a decreased pathologic response in patients receiving NACT for breast cancer.Conclusion: Our results show that a high BMI and HR positivity are associated with a poor response to neoadjuvant chemotherapy in Turkish patients with breast cancer. The findings presented in this study may guide novel studies to examine the NACT response in obese patients with and without insulin resistance.