Activation of blood coagulation and fibrinolysis has previously been detected in stroke patients. It is unknown, however, what factors contribute to the acceleration of coagulation reactions, especially in cases where no obvious predisposing factors exist. We therefore postulated and tested the hypothesis that in such patients monocytes may trigger the pathway leading to thrombosis by expressing tissue factor (TF). TF antigen was determined in 48 patients and 40 controls by flow cytometry using an indirect immunofluorescent technique. TF antigen expression was significantly increased on monocytes in young stroke patients in both the acute (p < 0.01) and chronic (p < 0.05) phases of the disease. The TF antigen also possessed functional activity, quantitated by a one-stage clotting assay. TF expression on monocytes was not associated with an elevation in C-reactive protein values. In both acute and chronic phases, blood coagulation activation markers, e.g. the thrombin-antithrombin complex and F₁₊₂ fragments, were significantly elevated. However, in the acute phase D-dimer levels were similar to those in controls and were only elevated in the chronic phase of the disease (p < 0.05). In conclusion, in cerebral ischemia TF expression on monocytes suggests enhanced activation of blood coagulation and subsequent fibrinolysis.