© Journal of Gastrointestinal Oncology. All rights reserved. Background: The impact of increased body mass index (BMI) on clinical outcomes in locoregional rectal cancer is unknown. Methods: This is a retrospective cohort study which included 453 consecutive rectal cancer patients undergoing definitive treatment, with confirmed stage I, II or III rectal adenocarcinoma. The association of BMI at diagnosis with overall survival (OS), cancer specific survival (CSS) and disease-free survival (DFS) was explored, controlling for key covariates using multivariable analyses. BMI as defined by the World Health Organization (WHO) is as follows: BMI <18.5-underweight; 18.5-24.9-normal; 25.0-29.9-pre-obesity; >30-obese. Results: Overweight and obese patients had significantly better OS than underweight/normal weight patients (5-year OS 80% for overweight, 77% for obese, and 65% for underweight/normal weight patients, P=0.02). High BMI (>25) was significantly associated with improved OS in univariate [0.62 (0.4-0.8) P=0.007] and multivariable [0.65 (0.4-0.9) P=0.023] analyses. When stratified by stage, high BMI was associated with improved OS in stage III patients (P=0.0009), but not stage II (P=0.21) or stage I (0.54). High BMI was also significantly associated with improved CSS in univariate (HR 0.62, P=0.048) and multivariable analyses (HR 0.58, P=0.03). Conclusions: In our study a BMI greater than 25 is significantly associated with a longer OS and CSS in patients with locoregional rectal cancer. These findings may be due to the reduced metabolic capacity for non-obese patients to deal with rectal cancer treatment as well as the burden of disease, however further research is needed to evaluate this.