Purpose: Obesity increases the risk of death from many malignancies, including non-Hodgkin's lymphoma (NHL). In diffuse large B-cell lymphoma (DLBCL), the most common form of NHL, the association between body mass index (BMI) at diagnosis and survival is unclear. Patients and Methods: We evaluated the association between BMI at diagnosis and overall survival in a retrospective cohort of 2,534 United States veterans diagnosed with DLBCL between October 1, 1998 and December 31, 2008. Cox modeling was used to control for patient- and disease-related prognostic variables. Results: Mean age at diagnosis was 68 years (range, 20 to 100 years); 64% of patients were overweight (BMI, 25 to < 30) or obese (BMI, ≥ 30). Obese patients were significantly younger, had significantly fewer B symptoms, and trended toward lower-stage disease, compared with other BMI groups. Cox analysis showed reduced mortality in overweight and obese patients (overweight: hazard ratio [HR], 0.73; 95% CI, 0.65 to 0.83; obese: HR, 0.68; 95% CI, 0.58 to 0.80), compared with normal-weight patients (BMI, 18.5 to < 25). Treatment during the rituximab era reduced the risk of death without affecting the association between BMI and survival. Disease-related weight loss occurred in 29% of patients with weight data 1 year before diagnosis. Cox analysis based on BMI 1 year before diagnosis continued to demonstrate reduced risk of death in overweight and obese patients. Conclusion: Being overweight or obese at the time of DLBCL diagnosis is associated with improved overall survival. Understanding the mechanisms responsible for this association will require further study.