A 64-year-old woman with a monoclonal gammopathy was admitted to Nagoya National Hospital with the complaint of occasional hemoptysis. On examination, there was no hepatosplenomegaly or no lymphadenopathy. The hemoglobin was 10.1 g/dl; platelets 22.5 X 10(4)/microliters; white blood cells 4.9 X 10(3)/microliters, with 4% of atypical lymphocytes. Immunoglobulin analysis of the serum by immunodiffusion revealed an IgG of 1,459 mg/dl, an IgA of 219 mg/dl, and an IgM of 5,091 mg/dl. Serum viscosity was 4.9. Serum immunoelectrophoresis demonstrated atypical precipitant arcs reacting with mu and kappa antisera. Urine immunoelectrophoresis showed a positive reaction against kappa antiserum. Radiologic studies of the bones revealed generalized osteoporosis with multiple punched out lesions of the skull. Thirty percent of bone marrow nucleated cells was atypical plasma cells, the presence of which was verified by electron microscopy. Although they were positive mainly for cytoplasmic mu and kappa chains by immunoperoxidase studies, cells positive for gamma, alpha, or lambda chains were occasionally found, indicating that normal immunoglobulin synthesis was not suppressed in this case of IgM myeloma.