Antiphospholipid antibodies (aPL) are a heterogeneous group of autoantibodies, detected in the sera of patients with both autoimmune and various non-autoimmune diseases. They are also detected in subjects with no overt underlying disease - the primary antiphosphilipid syndrome (PAPS). High titers of APL are associated with arterial and venous thrombosis, recurrent fetal loss and thrombocytopenia. There have been many suggestions explaining the potential mechanisms of the procoagulant effect of aPL. These include endothelial cell (EC) activation; increased adhesion molecule expression; inhibition of EC prostacyclin release, increased leucocyte adhesion to EC, downregulation of thrombomodulin expression. APL induce the procoagulant activity of monocytes via increased tissue factor expression and directly stimulate platelet hyperactivity with resultant production of enhanced amounts of the proaggregatory molecule of TXA2. In vitro studies show that prepro-endothelin-1 mRNA is induced by human monoclonal anticardiolipin antibodies and this might contribute to vasospasm, and, ultimately, to arterial occlusion. The hypercoagulable state in APS patients is associated with alterations in the protein C/S pathway. It is suggested that aPL may impair the protein C anticoagulant system. Acquired protein C and protein S deficiency is described in patients with APS. Beta2- glycoprotein I, (Beta2-GPI) a natural anticoagulant, is involved in the regulation of protein S anticoagulant activity by preventing the binding of protein S to C4b-binding protein. APL were shown to inhibit this effect of Beta2- GP I. As the group of aPL is very heterogeneous, it is unlikely that a single mechanism is responsible for the thrombogenic activity of all aPLs associated with thrombosis.