Lupus-like anticoagulant (LLAC) is the term used to identify an acquired inhibitor (1) of blood coagulation, first described by Conley and Hartmann in patients with systemic lupus erythematosus (SLE) (2). A circulating “lupus-like” anticoagulant has been detected in 5-10% of all SLE patients. A lupus anticoagulant is also found in other diseases including immunologic, oncologic, gynecologic, and neurologic disorders (3-12). In autoimmune diseases a lupus anticoagulant may occur in families, suggesting a genetic predisposition (13-15). Methods now available can distinguish between lupus anticoagulant and other inhibitors of coagulation (i.e., agonist factor VIII, IX, X, XI, and XII) occurring in SLE (16-19) and, more generally, in autoimmune diseases. In fact, in mixtures of LLAC plasma and normal plasma an immediate inhibition can be observed which is not increased by prolonging the incubation period, in contrast to the gradual time-dependent inhibition that occurs in patients with other types of coagulat...