One hundred and fifty-seven HIV seropositive patients were included in a prospective study of coagulation parameters. Activated partial thromboplastin time, prothrombin time, thrombin time and specific factor assays of the intrinsic pathway were performed using standard techniques. The tissue thromboplastin inhibition test and antiphospholipid antibodies were used to establish the presence of circulating lupus anticoagulant. Among the 46 patients with a prolonged activated partial thromboplastin time, an anti-prothrombinase was present in 33. Of the 111 patients with a normal activated partial thromboplastin time, anti-prothrombinase was present in 51. Circulating lupus anticoagulant seems to be common in HIV seropositive patients, since it was found in 84 patients (53.5%). Our findings confirm that the presence of circulating anticoagulants is not particularly associated with opportunistic infections or the development of the disease. It is possible that these inhibitors could be mediated by anti-phospholipid antibodies. In HIV seropositive patients, defective T cell regulation of B cells leads to polyclonal hypergammaglobulinemia. These antibodies may be directed against endogenous or exogenous phospholipids.