A patient with acute myelomonocytic leukemia was found to have IgG paraprotein on serum electrophoresis Bence Jones K proteinuria and increased plasma cells (30%) on marrow examination. The simultaneous occurrence of the two diseases was well documented by cytochemical immunological and electron-microscopic findings. Bone marrow chromosome investigations showed an abnormal karyotype: hypodiploidy was prevalent and marker chromosomes were present. A possible relationship between acute leukemia and multiple myeloma is discussed.