The aim of the study was to investigate whether interleukin-10 (IL-10) genetic polymorphisms influence this cytokine production as well as the incidence and outcome of diffuse large B-cell lymphoma (DLBCL). The frequency of IL-10(-1082G) allele was found to be higher in 199 patients with DLBCL as compared with 112 control subjects (0.47 versus 0.39, P =.043). Increased serum levels of IL-10 were associated with adverse prognostic factors and poor DLBCL outcome. The frequencies of IL-10(-819T) and IL-10(-592A) alleles were lower in patients with elevated IL-10 serum levels (0.155 versus 0.32, P =.14). As compared with patients carrying the IL-10(-1082AA) genotype, patients with the IL-10(-1082G) allele (IL-10(-1082GG/GA) genotypes) had higher complete remission rate (78% [confidence interval (CI), 71%-85%] versus 65% [CI, 52%-78%], P =.07), 5-year freedom from progression (FFP) (60% [CI, 52%-68%] versus 40% [CI, 27%-53%], P =.013), and overall survival (OS) (63% [CI, 55%-71%] versus 33% [CI, 20%-45%], P =.0009). Among factors of the International Prognostic Index, IL-10(-1082G) allele remained an independent variable, predicting longer freedom from progression (FFP) (RR [relative risk] =.76, P =.00035) and OS (RR =.78, P =.0015). These results indicate that IL-10 production contributes to the clinical course of DLBCL and that this phenomenon involves a substantial genetic component.