Long-term anticoagulation is widely used for secondary thromboprophylaxis in the antiphospholipid syndrome (APS) due to the high risk of recurrent events. Currently anticoagulation with vitamin K antagonists (VKAs) is the standard of care but have unpredictable pharmacodynamic properties that requiere monitoring for dose adjustment. Rivaroxaban, an orally active direct factor Xa inhibitor, has been shown to be effective and safe compared with warfarin for the treatment of venous thromboembolism and non valvular atrial fibrillation in major RCTs. No studies had been published in APS.The aim of the study is to investigate the efficacy and safety of rivaroxaban in preventing recurrent thrombosis in patients with APS compared with acenocumarol