Serum syndecan-1 was investigated in 189 patients with newly diagnosed monoclonal proteinemia [the diagnoses were multiple myeloma (66), monoclonal gammopathies of undetermined significance (MGUS; n=54), provisional MGUS (no bone marrow examination performed; n=69)] and 36 controls. Syndecan-1 levels ranged widely between all diagnostic categories and were of limited discriminatory value (sensitivity 68%, specificity 78%) in patients with newly diagnosed monoclonal proteinemia.