Thrombolytic treatment of acute myocardial infarction proved to be able to restore infarct artery patency and to decrease hospital mortality. The number of bleeding complications have remained at an acceptably low level, however some thromboembolic complications occurring during the first week following thrombolytic therapy have been recently observed. Signs of increased in vivo platelet activation (by measuring beta-thromboglobulin and thromboxane metabolite levels) and endothelial damage (Willebrand-factor estimations) could have been detected in our patients treated with brief high dose intravenous streptokinase, altogether with diminished antithrombin III and protein C antigen levels and activity. Intravenous streptokinase treatment of acute myocardial infarction might be able to cause thrombotic haemostatic alterations, which require meticulous haemostasis monitoring and early, correct antithrombiotic therapy.