A condition characterized by hypercoagulability associated with high blood levels of IgG antibodies against phospholipids, which are a major component of cell membranes. Many affected patients have a systemic autoimmune disease, such as systemic lupus erythematosus, but others present only with a history of frequent arterial and venous thrombi or pregnancy loss. Recent evidence suggests that antiphospholipid antibodies play a role in approx. 20% of strokes, especially in patients who do not have common risk factors for stroke. Antiphospholipid antibodies include lupus anticoagulant and anticardiolipins; the presence of the latter causes these patients to test positive for syphilis.