Background: The impact of patient body habitus and sex on outcomes in diffuse large B-cell lymphoma (DLBCL) remains controversial. We investigated the impact of body mass index (BMI), body surface area (BSA), age, and sex on clinical outcomes in patients with DLBCL treated in the rituximab era. Patients and Methods: Patients with de novo DLBCL (n=1,386) diagnosed between June 2000 and December 2010 treated with rituximab-containing chemotherapy were identified from the NCCN Oncology Outcomes Database for Non-Hodgkin's Lymphoma. Progression-free survival (PFS) and overall survival (OS) at 3 years were analyzed based on sex, age, and baseline BMI/BSA. Results: High BMI was associated with a lower risk of disease progression or death than low or normal BMI, whereas male sex was associated with poor clinical outcomes, especially among elderly patients (age >60 years). Compared with elderly women, elderly men experienced worse PFS (3-year hazard ratio [HR], 1.5) and OS (3-year HR, 1.6), but these differences diminished with increases in BMI and BSA. In multivariable analysis, normal BMI compared with high BMI was independently associated with poor outcomes (3-year PFS HR, 1.5; OS HR, 1.6) after adjusting for sex. Notably, only 13% of elderly men had BMI less than 25 kg/m2 and only 26% had BSA less than 2 m2. Conclusions: Analysis of unselected patients with DLBCL treated with rituximab-containing chemotherapy confirmed an age-dependent disadvantage to male sex in treatment outcomes, but this effect is abrogated by higher levels of BMI and BSA in most North American men.