BACKGROUND: In the acute phase of myocardial infarction, the hemostatic mechanism is known to be activated. However, it remains unclear whether increased activity of the hemostatic mechanism is only a marker of the acute thrombotic episode or precedes its appearance. It is also inapparent whether a hypercoagulable state persist for a prolonged period after the apparent resolution of these disorders. METHODS: In a group of 23 patients with acute myocardial infarction who received fibrinolytic therapy with urokinase(group A) or tPA(group B), the plasma level of fibrinogen, antithrombin compared to those of the 10 normal controls. RESULTS: The plasme level of fibrinogen was significantly decreased in both group A and B before and 4 to 24 hours after thrombolytic therapy compared to that of normal controls. But it was increased 7 to 14 days after thrombolytic therapy. In a few of the patients, the plasma level of FDP and D-dimer were positive before thrombolytic therapy and in the most patients they were positive 4 hours after thrombolytic therapy. The plasma level of AT-III was significantly increased in both group A and B before thrombolytic therapy compared with that of normal controls, but, after thrombolytic therapy, there was no significant change in its level. CONCLUSIONS: In the patients with acute myocardial infarction, the thrombolysis occurred before thrombolytic therapy and it lasted for 24 hours after thrombolytic therapy.