The aim of this study was to investigate whether failure of thrombolytic treatment might be due to inhibition of fibrinolysis by high lipoprotein(a) levels. Fifty-eight patients with acute myocardial infarction were treated intravenously within 4 h after onset of symptoms with anistreplase (30 units) and heparin (30,000 IU/24 h). Blood samples for measurement of coagulation parameters were taken before and 1.5 h after treatment. Coronary angiography was performed after 48 h. Levels of lipoprotein(a) were measured 6 months after discharge from hospital. The patency rate was 74% (43/58). Median lipoprotein(a) levels were not different between the patients with a patent and those with a non-patent vessel (10 and 8 mg/dl, respectively), however, in patients with a non-patent infarct-related vessel, a significant inverse correlation was found between the lipoprotein(a) level and the decrease of plasminogen in the first 1.5 h after treatment. It is concluded that high lipoprotein(a) levels, although not directly associated with a poor outcome of anistreplase therapy, might contribute to insufficient fibrinolysis in patients with a non-patent infarct-related vessel.