In 36 patients suffering from systemic lupus erythematosus (SLE), lupus anticoagulant (LA), as assessed by aPTT and diluted aPTT, and anticardiolipin antibodies (aCL) were studied. 14 patients, had a clinical history complicated by thrombosis and/or miscarriages. Among patients with thrombosis LA was positive in 42% and in 100% of patients when assessed by aPTT and diluted aPTT respectively; aCL were positive in 85.7% of patients. Among patients without a clinical history of thrombosis, 1 had prolonged aPTT, 3 had prolonged diluted aPTT and 5 had aCL positivity. Diluted aPTT was more sensitive than aPTT and aCL (p less than 0.01) to thrombosis and miscarriages; specificity to thrombosis and miscarriages ranged from 77.3% for aCL and 86.4% for diluted aPTT to 95.5% for aPTT but not significant differences were found. The study suggests that LA, as assessed by a sensitive test like diluted aPTT, is strongly associated to thrombosis and should therefore be considered an important risk factor.