CD5+ diffuse large B-cell lymphoma(CD5+DLBCL) is known to have different characteristics than CD5-DLBCL and mantle cell lymphoma(MCL). 9 patients with CD5+DLBCL were reviewed, and the results were compared with those of 8 CD5-DLBCL and 3 cyclin D1+MCL patients. CD5+DLBCL was more closely related to many aggressive clinical features or parameters than CD5-DLBCL: 67% of the patients were older than 60 years, 67% with performance status > or = 2, 89% with serum lactate dehydrogenase level higher than normal, 78% with stage III/IV disease at diagnosis, and 78% with more than one extranodal lesion. The overall International Prognostic Index score for the patients with CD5+DLBCL was thus significantly higher than that for those with CD5-DLBCL. Immunophenotypically, CD5+DLBCL was characterized by CD5+CD10-CD19+CD20+CD21-CD23-cyclin D1-phenotype and the predominant expression of surface IgM. Of particular interest is that the survival rate of CD5+DLBCL patients was significantly inferior to that of patients with CD5-DLBCL. To further characterize CD5+DLBCL, we semi-quantified the CD5 expression in DLBCL cells. Our findings suggest that CD5+DLBCL may constitute a unique subgroup of DLBCL and, moreover, CD5+DLBCL may consist of several subgroups.