The lupus anticoagulant is usually found in the plasma of patients with systemic lupus erythematosus. Lupus anticoagulants are antibodies to phospholipids and probably to phosphodiester-linked phosphate groups. A high frequency of thrombotic events in patients with lupus anticoagulant has been reported. Nevertheless the pathogenesis of thrombosis in these patients remains unknown. Endothelium which plays a key role in the antithrombogenic-thrombogenic balance could be a target for the lupus anticoagulant and alterations of some endothelial-cell functions could be responsible for the thrombotic events. The effects of the lupus anticoagulant on the phospholipids of the protein C-thrombomodulin complex may be important although evidence of such a reaction in vivo is awaited.