The aim of this study is to investigate the utility of the pre-operative systemic immune inflammation index (SII) as an independent prognostic factor in patients operated on for breast cancer. The clinical, pathological and laboratory data of 112 patients who were operated for breast cancer in Sivas Cumhuriyet University Surgical Oncology Department between January 1, 2014 and January 1, 2020 were retrospectively analyzed. Preoperative systemic immune-inflammation index (SII) was calculated. The SII cut-off value was set at 600. The SII value was compared with ca 15-3, histological grade, TNM classification, and tumor size which is an independent prognostic factor for breast cancer. All patients were women. The average age of the patients was calculated as 58.5 years. There was a significant relationship between SII and Ca 15.3, histological grade, tumor size, and anatomical stage obtained by TNM classification. The increase in the systemic immune-inflammation index (SII) as other prognostic values increase in our study suggests that this value can be used as an independent prognostic factor. We think that SII can be used as a prognostic factor in the clinic because it does not take much time to calculate it, it is easily accessible and its cost is low.