An autopsy case of multiple myeloma (MM) with marked elevation of serum IgM as an M-component, diagnosed as IgM myeloma, was described. Clinically, multiple bone lesions were found but neither lymphadenopathy, hepatosplenomegaly nor tumor mass formation in the gastrointestinal tract. Pathologically, the tumor cells proliferating in the bone lesions were mainly comprised of myeloma cells closely resembling mature plasma cells, though they contained immature cells as a minor cell population. At autopsy, slight infiltration of myeloma cells was confirmed in the lymph nodes, spleen and both kidneys, which is considered to be a terminal phenomenon that occurred in the clinical course of the case. From these findings, it appears reasonable that, though a monoclonal increase of serum IgM was found, this case differs from Waldenström's macroglobulinemia (WM) both clinically and pathologically and is rather situated within the category of MM. The occurrence of such a case is extremely rare, only 28 cases being reported in the literature, including 13 autopsy cases. The existence of IgM myeloma in the category of MM should be emphasized as a disorder distinct from WM.