A 71-year-old man was found to have chronic lymphocytic leukemia (CLL) and diffuse large cell lymphoma (DLC) simultaneously and was diagnosed as Richter's syndrome. The CLL had mu lambda surface immunoglobulin (sIg) whereas the DLC had mu kappa sIg. However, the immunoglobulin (Ig) gene rearrangement and surface marker analysis demonstrated that both CLL and DLC had identical rearrangement patterns of the Ig heavy chain (IgH) and identical surface markers CD5+, CD19+, and CD20+. These facts imply that in this case the two malignancies are of single clonal origin initially, and that different sIg of CLL and DLC do not, therefore, necessarily indicate the biclonality of these malignancies. The origin of DLC in Richter's syndrome remains controversial. This case suggests difficulty in concluding the biclonality of these malignancies. For a conclusion on clonality to be definitive, there is a need for cloning and nucleotide sequencing of rearranged Ig genes in more patients with Richter's syndrome.