The antiphospholipid antibody syndrome (APS) is a syndrome associated with excessive blood clotting (thrombosis). APS is among the most common cause of heart attack and stroke in patients under the age of 50 and is particularly prevalent in patients with autoimmune conditions. Patients with APS and prior thrombosis require lifelong anticoagulant therapy to prevent recurrent clots; such therapy is currently provided with warfarin. Warfarin requires frequent bloodwork monitoring, and many medications or foods can alter its effect, which can put people either at increased risk for clotting or bleeding. Rivaroxaban is a new mediation that prevents blood clots that does not require bloodwork monitoring and that has fewer interactions. This study is a pilot feasibility study which will: 1) examine our ability to identify 150 eligible APS patients; 2) measure our ability to obtain consent from 135 of these patients; and 3) test our hypothesis that we can obtain 95% compliance with daily rivaroxaban administration. The investigators propose to treat eligible patients with rivaroxaban 20 mg once daily. Patients will be followed for a minimum of one year and their rates of bleeding and thrombosis will be monitored as secondary outcome measures.

Inclusion Criteria:

• Prior objectively-confirmed venous thrombosis, irrespective of history of arterial thrombosis
• Two or more prior positive APS serological evaluations at least 12 weeks apart
• Current treatment with warfarin administered to achieve an INR of 2.0 to 3.0, rivaroxaban or dabigatran at any dose currently used for secondary prophylaxis of thrombosis, or short term therapeutic dose LMWH (for example, for the treatment of recently diagnosed deep vein thrombosis). Patients not currently receiving anticoagulation but in whom anticoagulation is mandated, may also be enrolled if a 20 mg rivaroxaban dose would be appropriate

Exclusion Criteria:

• Prior recurrent thrombosis while taking warfarin with a demonstrated INR of 2.0 to 3.0, or prior recurrent thrombosis while receiving dabigatran or rivaroxaban
• History of isolated arterial thrombosis (no history of venous thrombosis) pending CTA approval by Health Canada
• Need for continued treatment with both aspirin (irrespective of dose) AND clopidogrel
• Pregnancy or planned pregnancy during the study period; women who may become pregnant will be required to utilize reliable contraceptive measures while on study drug
• Chronic kidney disease with calculated GFR < 30mL/min
• Geographic inaccessibility
• Age < 18 years
• Inability or failure to provide informed consent