The plasma levels of thrombomodulin in 54 patients with Behçet's disease (BD) and 20 healthy control subjects were studied. The mean thrombomodulin (TM) level was significantly higher in active BD patients than in inactive patients and healthy controls ( P<0.001 and <0.01, respectively). Plasma TM levels did not show a significant relation with clinical manifestations. Increased plasma TM levels are associated with active disease and may reflect the presence of endothelial cell activation and/or injury.