Antiphospholipid antibodies (aPL) have been associated with clinical conditions that involve arterial or venous thrombotic events and pregnancy morbidity including fetal loss and preeclampsia. These antibodies are detected by various functional tests for the lupus anticoagulant, the anticardiolipin ELISA, the anti-beta2-glycoprotein I ELISA, or ELISA tests for other aPL. The pathogenic mechanisms are poorly understood. A "2 hit" hypothesis has been entertained in which there is underlying vascular (endothelial) damage, and in the presence of an aPL, a thrombotic complication emerges. Although the role of immunologic processes and autoimmunity appears important, immunosuppressive therapy has not proven very effective. Treatment options are limited to antiplatelet therapy (primarily for arterial events) and anticoagulation (with coumadin, heparin, or low molecular weight heparins) because of lack of understanding of the inciting factors and the pathogenesis of the process.