The differential diagnosis between multiple myeloma (MM) and Waldenström's macroglobulinemia (WM) is generally well defined. Consistent with a diagnosis of MM is the presence of a non-IgM monoclonal gammopathy associated to multiple osteolytic lesions and plasma cell infiltration of the bone marrow. Characteristic of WM is the presence of an IgM monoclonal gammopathy associated to lymphoadenopathy, hepatosplenomegaly, anemia and hyperviscosity syndrome in the presence of a monoclonal lymphoplasmacytoid proliferation in the bone marrow. Nonetheless, few cases of IgM myeloma have been reported that display clinico-pathologic features intermediate between MM and WM. Here, this study describes four of 317 (1.2%) patients with an IgM monoclonal gammopathy in whom the morphologic and clinical features were consistent with a diagnosis of IgM myeloma.