Thirty-four patients with an IgM M-component were evaluated for clinical presentation and course, laboratory data, and histologic findings, An attempt was made to ignore the presence of the IgM M-component and to assign each patient to one of the following categories: Waldenstrom's macroglobulinemia, IgM myeloma, Hodgkin's disease, lymphoma, chronic lymphocytic leukemia, chronic lymphosarcoma cell leukemia, and IgM M-components not associated with an identifiable lymphoproliferative disorder (“benign” M-component). Although transitional forms occasionally occurred, most patients could be readily categorized. The patients with lymphoma, Hodgkin's disease, chronic lymphocytic leukemia, and chronic lymphosarcoma cell leukemia did not appear to behave differently from patients with these disorders who did not have serum IgM M-component. Both for descriptive convenience and for clinical management, continued attempts should be made to separate patients with IgM M-components according to their underlying conditions.