Obesity is associated with altered immune and inflammatory responses and it may therefore influence the risk of non-Hodgkin's lymphoma. However, epidemiologic findings on obesity in relation to non-Hodgkin's lymphoma have been inconsistent. We conducted a meta-analysis to summarize the epidemiologic evidence on the association between excess body weight and risk of non-Hodgkin's lymphoma. Relevant studies were identified by searching MEDLINE (1966 to February 2007) and the reference lists of retrieved publications. We included cohort and case–control studies that reported relative risk (RR) estimates with 95% confidence intervals (CIs) for the association of body mass index (BMI) with non-Hodgkin's lymphoma incidence or mortality. A random-effects model was used to combine results from individual studies. Sixteen studies (10 cohorts and 6 case–control studies), with 21,720 cases, met the inclusion criteria. Compared to individuals of normal weight (BMI < 25.0 kg/m2), the summary RRs of non-Hodgkin's lymphoma were 1.07 (95% CI, 1.01–1.14) for overweight individuals (BMI between 25 and 30 kg/m2) and 1.20 (95% CI, 1.07–1.34) for those who were obese (BMI ≥≥≥≥ 30.0 kg/m2). Meta-analysis stratified by histologic subtypes showed that obesity was associated with a statistically significant increased risk of diffuse large B-cell lymphoma (RR, 1.40; 95% CI, 1.18–1.66; n = 6 studies) but not of follicular lymphoma (RR, 1.10; 95% CI, 0.82–1.47; n = 6 studies) or small lymphocytic lymphoma/chronic lymphocytic leukemia (RR, 0.95; 95% CI, 0.76–1.20; n = 3 studies). These findings indicate that excess body weight is associated with an increased risk of non-Hodgkin's lymphoma, especially of diffuse large B-cell lymphoma. © 2007 Wiley-Liss, Inc.