In a preliminary study, we have demonstrated that tissue factor (TF) is immunohistochemically stained in monocytes from patients with disseminated intravascular coagulation (DIC) but not from healthy volunteers, and that leukocytes from DIC patients induce enhanced activated factor X (FXa) generation in the presence of a mixture of FVIIa, FX and Ca(2+). Then, TF mRNA levels in leukocytes were measured to evaluate the role of TF in the pathophysiology of various diseases. TF mRNA levels in leukocytes were low in healthy volunteers but they were significantly increased in various diseases, especially in patients with infectious diseases, solid cancer, and hematopoietic tumors. TF mRNA levels in leukocytes were significantly higher in patients with high levels of C reactive protein (CRP) than in those with low CRP. TF mRNA levels were significantly higher in patients with DIC than in those without DIC. TF mRNA levels were well correlated with TF antigens in plasma and leukocytes. These findings suggest that the expression of TF mRNA in leukocytes is increased in various diseases and that this may play an important role in hypercoagulability or DIC.