During recent years it has become evident that lymphoproliferative diseases of B-cell origin display preferential immunoglobulin (Ig) variable heavy chain (V_H) gene usage. For instance, the V_H1-69 and V_H4-34 genes were early found to be overexpressed in B-cell chronic lymphocytic leukemia (CLL) and other B-cell lymphomas. The implications of biased V_H gene usage have been speculated to be a result of stimulation of unknown antigens, which gives increased proliferation of B-cells with certain V_H gene configuration and consequently higher probability to undergo transformation. Thus, V_H gene usage may play a role in development of leukemias and lymphomas. Recently, we could confirm the over usage of the V_H1-69 and V_H4-34 genes in CLL, but a novel finding was that the V_H3-21 gene was preferentially utilized in CLL patients with mutated V_H genes. These V_H3-21⁺ Ig rearrangements showed molecular peculiarities such as shorter lengths of the third complementarity determining region (CDR) and had similar amino acid composition of their CDR3s, implicating recognition of the same antigen in individual tumors. Most of the V_H3-21⁺ patients also showed a predominance of λ chain expression and biased usage of 1 specific V_λ gene, V2-14. Furthermore, overall survival appeared to correlate with V_H3-21 usage and, regardless of V_H gene mutation status, V_H3-21⁺ patients had a poor outcome. All in all, it appears that V_H3-21 gene usage define a new entity of CLL. The remaining question now to be clarified is if antigen(s) actually are involved in the pathogenesis of V_H3-21⁺ CLL.