In a woman with chronic lymphocytic leukemia (CLL), a maxillary plasmacytoma developed after 8 years. The membrane-bound immunoglobulin of the leukemic lymphocytes, the cytoplasmic immunoglobulin of the plasma cells, the serum monoclonal protein and the urine Bence-Jones protein had the same heavy and light chains--mu kappa. This suggests that the leukemic cells transformed to plasma cells. This very rare event is a complete reversal of the more common transformation that occurs in CLL and manifested by de-differentiation. Only in one case out of the 20 previously reported patients with CLL and multiple myeloma there was evidence, like in the current case, of a common clonal origin of the two B-cell neoplasms.