De novo CD5+ diffuse large B-cell lymphoma (CD5+ DLBCL) is known to have phenotypically and genotypically different characteristics than CD5- DLBCL and mantle cell lymphoma (MCL). To further characterize CD5+ DLBCL, 109 patients with CD5+ DLBCL were reviewed, and the results were compared with those of 384 CD5- DLBCL and 128 cyclin D1+ MCL patients. Patients with CD5+ DLBCL showed a higher age distribution (median, 66 years; P =.0083) and a female predominance (male-female ratio, 49:60, P =.011) compared with those with CD5- DLBCL. CD5+ DLBCL was more closely associated with many aggressive clinical features or parameters than CD5- DLBCL: 69% older than 60 years (P =.039), 34% with performance status greater than 1 (P =.0016), 69% with serum lactate dehydrogenase level higher than normal (P <.0001), 62% with stage III/IV disease at diagnosis (P =.0023), 35% with more than one extranodal site (P =.023), and 40% with B symptoms (P =.0031). The overall International Prognostic Index score was thus significantly higher for the patients with CD5+ DLBCL than for those with CD5- DLBCL (P =.00005). The most frequent site of extranodal involvement was bone marrow (28%), a higher frequency than that for CD5- DLBCL (P <.0001) but lower than that for cyclin D1+ MCL (P =.0015). Histopathologically, CD5+ DLBCL showed centroblastic morphology except for 3 patients with immunoblastic disease, and interfollicular growth pattern (7%) and intravascular or intrasinusoidal infiltration (19%) were observed. Immunophenotypically, CD5+ DLBCL was characterized by a CD5+CD10-CD19+CD20+CD21-CD23- cyclin D1- phenotype and a predominance of surface IgMkappa. Of particular interest is that CD5+ DLBCL was characterized by a survival curve significantly inferior to that for patients with CD5- DLBCL (P =.0026). These findings suggest that CD5+ DLBCL may constitute a unique subgroup of DLBCL.