Plasminogen activator inhibitor 1 (PAI-1) is a glycoprotein, the primary inhibitor of t-PA and u-PA. It plays an essential role in controlling any excessive activation of fibrinolysis. It is present in plasma associated with vitronectin, in free form or associated with t-PA and in the alpha granules of platelets. Fibrinolysis corresponds to the solubilization of the fibrinous thrombus by plasmin, an enzyme originating from plasminogen adsorbed to fibrin. Plasminogen is activated by t-PA and u-Pa. PAI-1 by inhibiting plasminogen activators, controls the degradation of fibrinous thrombus. A decrease in fibrinolytic activity promotes the occurrence of thrombosis, while excessive fibrinolysis leads to hemorrhages.