We investigated the clonal relationship of malignant cells in a patient affected with both chronic lymphocytic leukemia (CLL) and multiple myeloma (MM). CLL cells and malignant plasma cells synthesized IgG1 kappa and IgA kappa molecules, respectively; these monoclonal Ig shared idiotypic determinants, providing evidence that a single clonal disease occurred in this patient. Furthermore, when leukemic CLL cells were driven to differentiate in vitro to immunoblasts and plasma cells, a switch from IgG to IgA occurred in a significant percentage of cells that were double producers. These data suggest that, in some circumstances, CLL leukemic B cells may reach a more mature state, leading to the occurrence of clinical MM.