Lupus anticoagulant (LA) prolongs clotting times in vitro, but in vivo leads to an increased risk of thromboembolic complications. LA is detected in many patients with lupus erythematosus but also in subjects without any autoimmune disease (primary antiphospholipid syndrome). We determined LA in 133 patients, mostly with lupus erythematosus and other autoimmune diseases. Three screening and two confirmatory tests based on different mechanisms of clotting system activation were used. LA was found in 34 patients. DRVVT was the most useful test in detecting LA while kaolin clotting time was the least sensitive. Anticardiolipin antibodies (ACL) of IgG class were found in 41 patients studied (31%). They were most often detected in patient positive for LA (64%). ACL IgM were not associated with prolonged clotting times. Laboratory diagnosis of antiphospholipid syndrome is easy when the presence of LA is confirmed by two independent tests and in addition ACL would be detected. In other cases, when only single tests are positive, it is suggested to repeat diagnostic procedure at least once.