The presence of a monoclonal paraprotein in the serum or urine raises the possibility of myeloma or lymphoma/leukemia. Yet, in a significant proportion of individuals with serum paraproteins, particularly those with low levels of paraprotein, clinical and routine bone marrow evaluation is not diagnostic of an underlying neoplasm. The purpose of this study was to define the pathologic basis for macroglobulinemia in patients whose routine bone marrow biopsies were not diagnostic of a lymphoplasmacytic neoplasm. We used immunofluorescence microscopy and flow cytometry of cell suspensions prepared from aspirated marrow, as well as immunohistochemistry of core biopsies, to perform immunopathologic evaluations of the bone marrow from 16 such patients. Seven individuals without a monoclonal serum paraprotein, who were similarly studied, served as controls. Our data indicate that 13 of the 16 patients with monoclonal serum IgM paraproteins whose routine bone marrow biopsies were normal or showed nondiagnostic changes morphologically had a dispersed monotypic B lineage population of concordant immunoglobulin heavy and light chain type in the bone marrow. The immunophenotype of these cells spanned the range from mature B cell to plasmacytoid B cell to plasma cell. In four of these 13 patients a diagnosis of lymphoplasmacytic lymphoma could be made on the basis of greater than or equal to 20% monoclonal B lineage cells among bone marrow mononuclear cells.