The association between obesity and survival in non-Hodgkin lymphoma is unclear. Using the Ontario Cancer Registry we conducted a retrospective analysis of incident cases of aggressive-histology B-cell lymphoma treated with a rituximab-containing regimen with curative intent between 2008-2016. 6246 patients were included. On multivariable analysis the rate of all-cause mortality was lower for the overweight body mass index (BMI 25-29.9 kg/m 2 ) (HR 0.85; 95%CI 0.77-0.95) and obese BMI (≥30 kg/m 2 ) (HR 0.75; 95%CI 0.67-0.85) groups compared to the normal weight group (18.5-24.9 kg/m 2 ). Binomial logistic regression analysis revealed a lower odds ratio (OR) of admission to hospital during treatment in the overweight (OR 0.84; 95%CI 0.75-0.95) compared to normal weight BMI group. In the largest cohort to date of aggressive-histology B-cell lymphoma patients treated with rituximab, increased BMI is associated with a survival advantage, and the magnitude of this effect increases from overweight to obese BMI.