BACKGROUND: Obese breast cancer patients have worse prognosis than normal weight patients, but the level at which obesity is prognostically unfavorable is unclear. This study aimed to investigate different effects of Body Mass Index (BMI) on prognosis disease-free survival and overall survivor of breast cancer patients. METHOD: This retrospective cohort study analyzed the medical records of breast cancer patients who sought treatment at Namazi hospital in Shiraz, Iran between 2014 and 2019. Three groups of patients were divided according to BMI. Menopausal status, BMI status, clinicopathological characteristics, treatment, and overall survival (OS), and disease free survival (DFS) were comprehensively evaluated. The World Health Organization (WHO) BMI classification was used to categorize patients into three groups: normal weight (BMI <
25.0 kg/m2), overweight (25.0 ≤ BMI <
30.0 kg/m2), and obese (BMI ≥ 30.0 kg/m2). RESULTS: Of the 7134 breast cancer patients, the majority (42.6%) were in 25-30 kg/m CONCLUSION: Obesity leads to worse DFS and OS in patients with BC and the effects of obesity on the breast cancer prognosis seem to be clinically related to menopausal status. Once validated, these results should be considered in the development of prevention programs.