We examined the immunoglobulin (Ig) heavy chain variable region genes (V(H) genes) used by leukemia cells of 1220 unrelated patients with chronic lymphocytic leukemia (CLL). We found 1188 (97%) expressed Ig encoded by a single Ig V(H) subgroup, the most common of which was V(H)3 (571 or 48.1%), followed by V(H)1 (319 or 26.8%) and V(H)4 (241 or 20.2%). Using allele-specific primers, we found 13.8% of all samples (n = 164) used one major V(H)1-69 allele, designated 51p1, 163 of which were not somatically mutated. For these cases, there was marked restriction in the structure of the Ig third complementarity determining regions (CDR3s), which were encoded by a small number of unmutated D and J(H) gene segments. Strikingly, 15 of the 163 cases had virtually identical CDR3s encoded by the second reading frame of D3-16 and J(H)3. Further analysis revealed that each of these 15 samples used the same unmutated Ig kappa light-chain gene, namely A27. These data reveal that approximately 1.3% (15/1220) of all patients had leukemia cells that expressed virtually identical Ig. This finding provides compelling evidence that the Ig expressed by CLL B cells are highly selected and not representative of the Ig expressed by naive B cells.