To define clinical and laboratory characteristics of the lupus anticoagulant (LA), we reviewed our experience (219 subjects). Subjects were divided into group A, those with the LA and the diagnosis of lupus erythematosus, group B, those with the LA but nonlupus diagnoses, and group C, those with drug-related lupus syndromes. The typical laboratory findings consisted of a prolonged and inhibited plasma clot time (an average of 1.9 times control time) which was proportionately more prolonged than the partial thromboplastin time or activated partial thromboplastin time (APTT) (average 1.3 times control). Ninety-eight percent had a prolonged plasma clot time and 94% had a prolonged partial thromboplastin time. The prothrombin and thrombin times were prolonged in 33 and 25% of subjects, respectively. Washed platelets shortened the APTT in the 22 subjects so tested. Monoclonal protein peaks were seen in 7% of patients. Seventeen episodes of bleeding were observed, but in all but one instance there was another hemostatic defect present. In the 18 patients who underwent major operations, there were no hemorrhagic complications. Fifty-eight episodes of thrombosis were observed with the same incidence in group A (25%) as in group B (26%). Bleeding is rare with the LA but thrombosis is common even without SLE and lupuslike syndromes. The plasma clot time in platelet-rich plasma is more prolonged, and in our experience, is more sensitive in detecting the lupus anticoagulant than is the partial thromboplastin time.