Plasma thrombomodulin (TM) levels were significantly elevated at disease onset in patients with thrombotic thrombocytopenic purpura (TTP) and disseminated intravascular coagulation (DIC), but was not in those with essential thrombocythemia and idiopathic thrombocytopenic purpura. However, in patients with TTP and DIC, TM levels decreased significantly after they achieved complete remission. In both TTP and DIC patients, plasma TM levels at onset in those with poor prognosis were higher than that in those with good prognosis. Among DIC patients, the plasma TM level was higher in those with organ failure than in those without, but there were no differences among patients with various underlying diseases associated with DIC. It is speculated that the plasma TM level reflects damage to vascular endothelial cells or organ failure and that it is useful in assessing prognosis for patients with DIC and TTP.