IgG-class anticardiolipin antibodies (IgG-ACA) were found in 25% of patients with myasthenia gravis. The prevalence and the level distribution were significantly different from those of a normal donor population (p<0.001). In myastcnic patients, IgG-ACA bound negatively charged, but not zwitteri-onic. phospholipids. They were significantly associated with the thymic abnormalities, thymoma and thymic hyperplasia, but not with various factors such as age, sex, antinuclear antibodies, severity of the disease and clinical thrombosis. The IgG-ACA levels did not correlate with titers of anti-acctylcholine receptor antibodies. Thus in Myasthenia Gravis, asymptomatic IgG-ACA could reflect an immune dysregulation under the influence of thymic alterations.