Antiphospholipid antibody syndrome is an acquired autoimmune disorder characterized by vascular thrombosis and/or recurrent pregnancy losses along with laboratory evidence of antiphospholipid antibodies. Anticoagulation rather than immunosuppression is the mainstay of treatment. Despite the effectiveness of oral anticoagulation for the prevention of recurrent thromboembolic episodes, thrombotic complications in the setting of apparently therapeutic oral anticoagulation have been observed; this may at times be due to difficulties in maintaining a consistently therapeutic level of anticoagulation. Low-molecular-weight heparin has been a useful alternative for long-term anticoagulation when there is difficulty in managing oral anticoagulant therapy and has the advantage of a consistent anticoagulant effect. In this report, we describe a woman with primary antiphospholipid antibody syndrome who developed extensive pulmonary embolism despite receiving a proven therapeutic dosage of low molecular weight heparin.