Impaired fibrinolytic function, mainly due to an elevation of plasma plasminogen activator inhibitor 1 concentration, is a common finding in patients with thrombotic disease. In patients subjected to hip surgery, preoperatively increased levels of plasminogen activator inhibitor 1 seem to be predictive of postoperative deep venous thrombosis. Several prospective studies of patients with angina pectoris or a past myocardial infarction have shown strong correlation between plasmatic plasminogen activator inhibitor 1 elevation and future cardiovascular events. However, before plasminogen activator inhibitor 1 can be regarded as a risk factor in the conventional epidemiologic sense, its relationship to myocardial infarction must be demonstrated in prospective studies of healthy populations. The regulation of plasminogen activator inhibitor 1 concentration in plasma is complex and at present not well understood. Multiple interactions with disturbances of both carbohydrate and lipoprotein metabolism are evident. Studies performed in cultured cells, transformed or natural, can be translated into human pathophysiology only with great caution. Both environmental and genetic factors seem to be of importance for the plasma plasminogen activator inhibitor 1 concentration. Platelet plasminogen activator inhibitor 1 seems to play a role in the resistance of platelet-rich thrombi to thrombolytic treatment.