We evaluated the prevalence end clinical significance of anti-cardiolipin antibodies measured by enzyme- linked immunosorbent assay in 96 sera obtained from the 48 patients with systemic lupus erythematosus. The results were as follows; 1) The prevalence of anti-cardiolipin antibodies was 56.3%. According to subclasses of anti-cardiolipin antibodies, the prevalence of IgG anti-cardiolipin antibodies was 45.8% and that of IgM anti-cardiolipin antibodies 27.1%. 2) The presence of anti-cardiolipin antibodies were well correlated with cutaneous vascultitis and thrombocytopenia, But they were not correlated with Raynaud phenomenon, neurologic involvement, thrombosis, habitual abortion or intrauterine death, and false positive test of VDRL. These results suggested that the detection and serial monitoring of anti-cardiolipin antibodies were helpful for the prediction of clinical manifestations and prevention of serious complications in systemic lupus erythematosus.