The correlation between the presence of lupus circulating anticoagulant (LCA) and the incidence of thromboembolic phenomena was evaluated in 66 systemic lupus erythematosus (SLE) patients. Our criteria for the presence of LCA included an elevated LCA index and a prolonged recalcification time. Thirty-two patients (48%) fulfilled these criteria (group A). The incidence of thromboembolic phenomena, recurrent abortions and involvement of the nervous system was higher in group A patients than in SLE patients without LCA (group B). Moreover, 16 patients of group A who exhibited also a positive thromboplastin inhibition test associated with a markedly elevated LCA index, manifested higher incidence of severe thromboembolic phenomena. Early detection of LCA has important therapeutic implications. We suggest that the presence of LCA should be recognized as one of the criteria for the diagnosis of SLE.