A 70-year-old woman was admitted for anemia, elevated serum total protein and a right axillary mass. Laboratory data showed monoclonal x IgM with a decrease in serum IgG and IgA levels. An occipital punched-out lesion was detected on a cranial X-ray. A tumor lesion was detected on chest X-ray and computed tomography. Biopsy specimen revealed plasmacytoma with cytoplasmic IgM. Bone marrow aspiration revealed an elevated plasma cell count. An immunophenotype analysis of the plasma cells showed positivity of cytoplasmic IgM, x, CD5, CD38, CD11a (LFA-1), CD44 (HCAM), CD49d (VLA-4) and CD54 (ICAM-1). From the above results, we diagnosed the patient as having IgM myeloma associated with plasmacytoma. Melphalan and prednisolone therapy were prescribed, their effect on the myeloma was short term, so we changed the chemotherapy to VAD (vincristine, adriamycin and dexamethasone), but this treatment had little effect. The patient developed bacterial pneumonia and died. IgM myeloma is a rare disease and reports of immunophenotype analysis are also rare. There is no case report of plasmacytoma associated with IgM myeloma.