DNA content of both bone marrow and peripheral blood mononuclear cells was measured by flow cytometric analysis in 46 patients with untreated multiple myeloma and 15 patients with benign monoclonal gammopathy to clarify further the incidence and clinical correlations of DNA aneuploidy. Aneuploidy was detected in the bone marrow of 25 multiple myeloma patients (54%) but in only one benign monoclonal gammopathy patient (7%), who developed multiple myeloma 34 months later. Thus DNA aneuploidy is considered rare in benign monoclonal gammopathy. In two multiple myeloma patients, DNA aneuploidy was detected also in blood, indicating circulating myeloma cells. The light chain of the M component was more frequently lambda in the diploid and kappa in the aneuploid group. Most of the patients with only light chain secretion were DNA aneuploid. Multiple myeloma patients with DNA hypodiploidy (7%), biclonal aneuploidy (4%), or DNA aneuploidy detectable in blood (4%) did not respond to therapy with melphalan and prednisone. Survival was not influenced by DNA content. No DNA aneuploidy was detected in the bone marrow or the peripheral blood of 26 patients with chronic lymphocytic leukemia or two patients with Waldenström's macroglobulinemia.