We report fifteen patients, thirteen with systemic lupus and two patients with a "lupus-like" illness who developed cerebral infarction. All fifteen patients were shown to have elevated anticardiolipin antibody levels using a newly devised solid phase radioimmunoassay. The lupus anticoagulant was detected in all eleven patients tested. It is proposed that anticardiolipin antibodies and the lupus anticoagulant make up a population of antiphospholipid antibodies capable of causing cerebral vascular injury and thrombosis resulting in cerebral infarction. These antibodies may also play a pathogenic role in autoimmune disorders other than lupus where cerebral thrombotic disease is a prominent feature.