Plasminogen activator inhibitors (PAIs) regulate plasminogen activation in normal and pathologic processes. Plasminogen activator inhibitor 1 (PAI-1) is the major physiologic inhibitor of both tissue-type and urokinase-type plasminogen activators. It is a highly regulated single-chain glycoprotein, whose overexpression in vivo impairs the fibrinolytic balance and correlates with thrombotic disorders. Recent clinical observations suggest an association between elevated plasma PAI-1 and symptomatic coronary artery occlusive disease or deep venous thrombosis. Recognition of the clinical relevance of PAIs and timely assessment of the fibrinolytic capacity in patients at risk may have therapeutic implications.