Clinical and serological features in SLE patients with arterial or venous thrombosis were studied. The subjects consisted of 140 patients with SLE who met the revised criteria for the classification of SLE by the American Rheumatism Association. Forty patients (29%) had arterial or venous thrombosis. Arterial thrombosis such as stroke was found in 30 patients, and venous thrombosis such as deep vein thrombosis was seen in 24 patients. Average age at the disease onset was 34.5 +/- 12.5 years old. Renal disorder was found as a clinical feature, and IgG anticardiolipin antibodies (aCL), IgG phospholipid-dependent anti-beta 2-glycoprotein I (beta 2-GPI) antibodies and IgG anti-Annexin V antibodies were identified as serological features in SLE patients with thrombosis. These patients were diagnosed as having antiphospholipid syndrome. It was necessary to perform primary prevention therapy as well as secondary prevention therapy. Multiple thrombotic events in the past history and sustained positive reactions of IgG aCL were suggested as predictors of recurrent thrombosis. These data indicated the clinical and serological characteristics in SLE patients with arterial or venous thrombosis.