Background: The aim of this study was to investigate the relationship of body mass index with overall and progression-free survival as well as other prognostic factors of breast cancer in patients with non-metastatic breast cancer. Materials and Methods: We retrospectively reviewed 456 patients diagnosed with breast cancer in the Radiation Oncology department of Kayseri Teaching Hospital between 2005 and 2013. We investigated relationship of body mass index with prognosis and other prognostic factors. Results: The study included 456 patients (447 women and 9 men). Mean age at presentation was 55.6 years. Of the cases, 96.9% underwent modified radical mastectomy and 95.0% received chemotherapy, while 82.4% received radiotherapy and 60.0% were given hormone therapy. Body mass index was >25 mg/kg2 in 343 cases. Five- and 10-years overall survival rates were 77% and 58% whereas progression-free survival rates were 65% and 49%, respectively. In univariate analyses, factors including stage (p=0.046), tumor diameter (p=0.001), lymph node metastasis (p=0.006) and body mass index (p=0.030) were found to be significantly associated with overall survival, while perinodal involvement was found to be significantly associated with progression-free survival (p=0.018). In multivariate analysis, stage (p=0.032; OR: 3.8; 95% CI: 1.1-13), tumor diameter (p<0.000; OR: 0.0; 95% CI: 0.0-0.3), lymph node metastasis (p=0.005; OR: 0.0; 95% CI: 0.0-0.5) and BMI (p=0.027; OR: 0.02; 95% CI: 0.0-0.8) remained as significantly associated with OS. Conclusions: In our study, it was seen that overall survival time was shorter in underweight and obese patients when compared to normal weight patients.