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/m(2)), 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/m(2)) and 1.20 (95% CI, 1.07-1.34) for those who were obese (BMI >/=>/=>/=>/= 30.0 kg/m(2)). 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.