Diffuse large B-cell lymphomas (DLBCL) are the most common type of aggressive lymphomas, with considerable heterogeneity in clinical presentation, molecular characteristics, and outcome. Previous studies have showed significant correlations between DNA methyltransferase (DNMT) overexpression and unfavorable prognosis in human cancers. Therefore, we investigated in this study the biological and prognostic significance of DNMT1, DNMT3a, and DNMT3b protein expression in DLBCL. DNA methyltransferase (DNMT) expression was analyzed by immunohistochemistry in 81 DLBCL cases and correlated with clinicopathological parameters. Kaplan-Meier curves were used to estimate survival rates, and the Cox proportional hazard regression model was used to evaluate the prognostic impact of DNMT expression. Our results showed that overexpression of DNMT1, DNMT3a, and DNMT3b were detected in 48%, 13%, and 45% of investigated cases, respectively. DNA methyltransferase 1 (DNMT1) and DNMT3b overexpression was significantly correlated with advanced clinical stages (P = 0.028 and P = 0.016, respectively). Moreover, concomitant expression of DNMT1 and DNMT3b was significantly correlated with resistance to treatment (P = 0.015). With regard to survival rates, although data was available only for 40 patients, DNMT3b overexpression was significantly correlated with shorter overall survival (P = 0.006) and progression-free survival (P = 0.016). Interestingly, multivariate analysis demonstrated that DNMT3b overexpression was an independent prognostic factor for predicting shortened overall survival (P = 0.004) and progression-free survival (P = 0.024). In conclusion, DNMT3b overexpression was identified as an independent prognostic factor for predicting shortened survival of patients with DLBCL and could be, therefore, useful in identifying patients who would benefit from aggressive therapy. (Cancer Sci 2010; 101: 1722-1730 D iffuse large B-cell lymphomas (DLBCL) are the most common form of aggressive lymphoma.(1,2) Although curable in the majority of cases with anthracycline-based combination chemotherapy and the monoclonal antibody rituximab, approximately 40% of patients with DLBCL will relapse after standard first-line therapy.(1,2) Several biological markers have been studied in attempting to identify high-risk patients, but none has proved to be completely effective, (2,3) and the international prognostic index (IPI) has remained the gold standard for predicting prognosis.(4) Recently, microarray gene expression studies have identified multiple genes of potential prognostic significance in DLBCL, and have led to the subdivision of DLBCL into two major biological categories based on presumed cell of origin: germinal centre B-cell like (GCB), and non-GCB ⁄ activated B-cell like.(5) However, the prognostic value of genetic markers in DLBCL remains controversial. The identification of new molecular prognostic markers would therefore improve the prognostic predictability and understanding of the clinical behavior of those lymphomas. (2,3) DNA me...