A factor XII dependent plasminogen activator has been described in human blood. In vitro its activity in plasma is mainly generated via kallikrein. Clinical data suggest long-term depression after myocardial infarction and relation to reinfarction. We were interested in FXII dependent plasminogen activator plasma levels during the acute phase of coronary or cerebral artery occlusion before therapeutic intervention. Plasma was obtained from 41 consecutive patients with myocardial infarction and from 40 patients with cerebral infarction immediately on admission to the hospital. Measurement was performed in dextran sulphate euglobulin fractions in the presence of neutralizing antibodies towards t-PA and u-PA. Plasma levels of FXII dependent fibrinolytic activity were found highly significantly reduced both in myocardial (18.3 (8.1-26.3); p = 0.003. Blood activator units/ml; median value; (25-75 interpercentile range)) and in cerebral infarction (14.4 (7.6-25.6); p < 0.001) as compared to controls (35.3 (27.1-46.2)), but without difference between patient groups. The FXII dependent plasminogen activator plasma levels were neither related to age, gender, smoking or hypertension nor to t-PA or PAI-1 plasma levels. Therefore we conclude that FXII dependent fibrinolytic activity plays an independent role in the acute phase of cerebral or coronary artery occlusion.