The antiphospholipid syndrome is defined as the occurrence of one or more episodes of vascular thrombosis and/or miscarriage together with antiphospholipid antibodies. Some of these antibodies can be detected via coagulation assays by prolonging phospholipids-dependent assays. These unspecific inhibitors are termed lupus anticoagulant, due to their first description by Conley and Hartmann in patients with disseminated lupus erythematosus. Lupus anticoagulants are now defined as acquired autoantibodies directed against a phospholipid-binding protein such as beta2-glycoprotein I or prothrombin. Because of this binding, lupus anticoagulants form bivalent complexes which slow down coagulation reactions in vitro by forming stable complexes on coagulation active phospholipids. In vivo, these complexes may result in cellular activation and cause thrombosis. Laboratory diagnosis for the lupus anticoagulant should be performed by a combination of tests, including phospholipid-dependent clotting assays, plasma mixing studies, and demonstration of the phospholipid-dependency of the inhibitory activity.