We report a case of an elderly woman with persistent lymphocytosis in whom flow cytometric immunophenotyping revealed 2 distinct clonal B-cell populations with different light chain restrictions. One clone was CD19+ (dim), CD5+ (bright), CD23+ (moderate), and kappa+; the other clone was CD19+ (moderate), CD5+ (bright), CD23+ (dim), and lambda+. We separated the 2 clones by flow cytometry from peripheral blood lymphocytes for further molecular and cytogenetic analysis. Polymerase chain reaction (PCR) analysis of the immunoglobulin heavy chain (IgH) genes revealed that the clones had bands of slightly different size. The PCR products of the framework 3 (FR3) region were cloned, and sequence analysis confirmed that each population had distinct, clone-specific IgH gene rearrangements. Fluorescence in situ hybridization (FISH) analysis revealed trisomy 12 in the lambda-restricted B-cell clone, whereas the k-restricted population had normal FISH patterns. Our results demonstrated that the immunophenotypically different cell populations originated from 2 separate clones.