Thrombopoietin (TPO) is the major regulator of growth and differentiation of megakaryocytes. Recent studies have shown that TPO may also act as an acute-phase reactant, and it has been suggested as a component of inflammatory reactions. In this study our objective was to investigate serum TPO levels in patients with rheumatoid arthritis, a complex chronic inflammatory disorder not uncommonly associated with thrombocytosis. Bloodstream TPO concentrations were assessed in 13 RA patients with platelet counts between 450 and 650 x 10(9)/1, 10 RA patients with platelet counts > 650 x 10(9)/1, 15 RA patients with normal platelet counts and 12 healthy controls. RA patients with normal platelet counts had TPO levels comparable with healthy controls. TPO concentrations in patients with mild thrombocytosis were significantly elevated, whereas patients with markedly increased thrombocyte counts had prominently decreased TPO levels. These results indicate that TPO seems to be associated with reactive thrombocytosis in RA patients with active disease. In patients with extremely increased thrombocytosis serum TPO levels might be regulated by increased platelet mass via receptor-mediated uptake and metabolism.