Metabolic syndrome is a clinical condition with a combination of multiple cardiac risk factors including obesity, insulin resistance, hypertriglyceridemia, low HDL, and hypertension. There is serious evidence that metabolic syndrome increases the risk of breast cancer. In this study, we aimed to define the frequency and determinants of metabolic syndrome in operated patients with stage I-III breast cancer. Operated patients with stage I-III breast cancer who admitted to our clinic between April 2009 and April 2018 were examined cross-sectionally. Metabolic syndrome was defined according to NCEP criteria. Metabolic syndrome criteria, demographic data, tumor size, grade, lymph node, estrogen-progesterone, HER2 status, and chemotherapy / endocrine treatment histories were obtained from patients and hospital records. 700 patients with a median age of 50.6 were analyzed. Tamoxifen was given to 194 patients and aromatase inhibitors were given to 240 patients. Any hormonal therapy was not given to 266 patients (new diagnosis and/ or triple-negative patients). Metabolic syndrome was observed in 43.1% of patients according to NCEP criteria Metabolic syndrome was found to be more frequent in the group receiving aromatase inhibitor than the group receiving tamoxifen (53.4% vs. 24.7%, p< 0.001). The frequency of metabolic syndrome was 48.2% in the group not receiving any hormonal treatment. Metabolic syndrome was more common in the postmenopausal patient group than the premenopausal group (49.0% vs. 26.1% p< 0.001). The incidence of metabolic syndrome was lower in patients with HER2 positive and a history of oral contraceptive use. (33.9% vs. 44.0% HER2, 0.11 and 37.7% vs. 40.8% oral contraceptive use history, p= 0.25). 75.1% of the patients had received adjuvant chemotherapy. There was no difference in the frequency of metabolic syndrome between the groups receiving and not receiving adjuvant chemotherapy. There was no statistically significant correlation between the presence of metabolic syndrome and estrogen/progesterone receptor status, tumor size, lymph node, stage, grade, hormone replacement therapy, chemotherapy, and smoking history. 43.1% of operated patients with stage I-III breast cancer had metabolic syndrome. Metabolic syndrome was more common in patients receiving adjuvant aromatase inhibitors, whereas less common in patients using oral contraceptives and having HER2 positive. These findings suggest that aromatase inhibitors may contribute to the development of the metabolic syndrome. Prospective studies are needed to explain this relationship.