IgM multiple myeloma (MM) and Waldenstrom's macroglobulinemia (WM) are two distinct hematologic entities with the common finding of an IgM monoclonal gammopathy. Distinguishing these two diagnoses is critical as the approach to therapy is different. A priori, we defined IgM MM as a symptomatic clonal plasma cell proliferative disorder characterized by an IgM monoclonal protein (regardless of size), 10% or more plasma cells on bone marrow biopsy, plus the presence of lytic bone lesions and/or translocation t(11;14). Twenty-one patients met this definition of IgM MM. All 21 patients had lytic bone lesions. Of the 16 evaluated with FISH, 6 (38%) demonstrated t(11;14). Median overall survival was 30 months, which is similar to non-IgM myeloma patients treated during this period and shorter than what would be expected for WM. In this, the largest series of patients with IgM MM, we describe the clinical features and prognosis of patient with IgM MM using a strict definition for the disease. The subset of patients without lytic lesions or t(11;14) but with immunophenotypic features suggestive of MM need further study. Am. J. Hematol., 2010. © 2010 Wiley-Liss, Inc.