A 71-year-old man was hospitalized in November, 1983 for a back pain and a diagnosis of multiple myeloma was made, based on the Bence Jones proteinuria, The serum M-component of a IgG-kappa type (3.3 g/dl), and plasmacytosis in the bone marrow (37%). Treatment consisted of melphalan and prednisolone. A blood count in March, 1986 revealed 6000/microliters of WBC with 30% of a blast form and 8% plasma cells, and 20,000/microliters of platelets. A bone marrow aspirate revealed that 14% were myeloblasts and 26% were plasma cells. Distinguishing the myeloblasts from the immature plasma cells in the peripheral blood proved difficult. Studies by electron microscopy and an immunological inspection of phenotypes were helpful in achieving a determination. A karyotypic analysis of the bone marrow cells indicated a hypodiploid cell population, a marker chromosome, and a karyotypic instability. These findings indicate that his multiple myeloma had undergone a leukemic change associated with acute myelogenous leukemia.