Abstract 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 diferent efects 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 classifcation 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/m2 . Menopausal status signifcantly were associated with obesity (P<0.001). The majority of patients were categorized as grade 2 and stage 2 according to the BMI categories (P=0.12, P=0.08, respectively). BMI categories regardless of menopausal status displayed increased 1, 3, and 5-year DFS and 5- year OS in stage 1 and increased 1, 3, and 5-year OS and 1 and 3-year DFS in stage 2. In stage 3, the risks of relapse and death were signifcantly decreased in all three groups of BMI patients with post-menopausal period. Conclusion Obesity leads to worse DFS and OS in patients with BC and the efects 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. Keywords Body mass index, Breast cancer, Survival, Prognosis

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