CD5(+) diffuse large B-cell lymphomas (DLBLs) have recently been described as a particular subgroup of DLBLs. Classical banding and interphase cytogenetic analyses targeting ATM, TP53, and P16(INK4a) genes and the D13S25 locus from 13 CD5(+) DLBLs were compared with 55 CD5(-) DLBLs. Additionally, analysis of somatic mutations of the immunoglobulin heavy chain variable region (IgVH) genes were performed in CD5(+) DLBLs. CD5(+) DLBLs were somatically mutated (7 of 8 cases) and were negative for t(11;14)(q13;q32) and t(14;18)(q32;q21), whereas t(3;14)(q27;q32) was found in only one tumor. Trisomy 3 and gains on chromosomes 16/16p and 18/18q were significantly overrepresented in CD5(+) DLBLs. No ATM deletions were detected. The prevalence of deletions at the D13S25 locus was significantly higher in CD5(+) DLBLs (4 of 12 [33%]) compared with CD5(-) DLBLs (4 of 42 [10%]), as were p16(INK4a) deletions (33% versus 8%). On the basis of these findings, CD5(+) DLBLs are likely to arise from the same progenitor cell as the mutated variant of CD5(+) lymphocytic lymphoma/B-cell chronic lymphocytic leukemia (B-CLL).