Antiphospholipid antibody is associated with a clinical syndrome of vascular thrombosis, thrombocytopenia, recurrent fetal loss, and livedo reticularis, whether or not a clinical diagnosis of systemic lupus erythematosus (SLE) coexists. A positive antiphospholipid antibody test is defined by enzyme-linked immunosorbent assay (ELISA) (antiphospholipid antibody itself) or by coagulation assay (lupus anticoagulant). These are similar but not identical antibodies. The test for syphilis is less closely related to the preceding two and is less regularly associated with clinical complications. The mechanism of action of either antiphospholipid antibody or lupus anticoagulant is as yet unknown. SLE-induced but not infection-induced antiphospholipid antibody has immunoglobulin G2 (IgG2) and IgG4 predominance. It recognizes all negatively charged phospholipids, but various physical characteristics of the phospholipids alter the recognition patterns. Treatment for the antiphospholipid antibody syndrome has not been clearly defined. Anticoagulation with aspirin, heparin, or warfarin is currently favored. A role for corticosteroid remains to be demonstrated.