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A lymphoma with the characteristic features of Hodgkin's disease (HD) occasionally develops in patients with B-cell chronic lymphocytic leukemia (CLL), and has been called Richter's syndrome with HD features. In such cases, large tumor cells have the morphological and immunophenotypic features of classical Hodgkin and Reed-Sternberg (H-RS) cells. However, it is not known whether the H-RS cells arise from transformation of the underlying CLL cells or from a different pathological process. We report herein a study of the clonal relationship between the CLL cells and the H-RS cells in three cases of Richter's syndrome with HD features by using a single cell assay. We isolated single CLL cells and H-RS cells from immunostained tissue sections by micromanipulation. The immunoglobulin heavy chain gene (IgH) complementarity determining region (CDR) III of each cell was amplified by the polymerase chain reaction (PCR). The products were then compared by gel electrophoresis and nucleotide sequencing. The IgH CDRIII sequences from the H-RS cells were identical to those from the CLL cells in two cases. In one case, the clonal relationship between the two types of cells could not be determined because PCR products could not be obtained from any of the H-RS cells. This study shows that the H-RS cells and the CLL cells belong to the same clonal population in some cases of Richter's syndrome with HD features. Furthermore, our findings indicate that mature B cells can undergo transformation to cells with the features of H-RS cells, in association with a cellular background typical of HD. This study also supports recent findings suggesting that the H-RS cells in classical HD are derived from transformed B cells.