We recently reported [J. Lipid Res. 42 (2001), 697; 43 (2002), 1486; 44 (2003), 716] that [beta2-glycoprotein I (beta2GPI) forms complexes with oxidized LDL (oxLDL) and autoantibodies against these complexes are present in patients with SLE and antiphospholipid syndrome (APS). The relationship of beta2GPI/oxLDL complexes and IgG autoantibodies against beta2GPI complexed with oxLig-1 (an oxLDL-derived ligand) with clinical manifestations of APS was studied in 150 APS and SLE patients. The beta2GPI/oxLDL levels of APS patients were similar to those of SLE patients without APS, but they were significantly higher than healthy individuals. There was no difference in the complex levels among the patients with arterial, venous thrombosis, or pregnancy morbidity. IgG anti-beta2GPI/oxLig-1 levels of APS were significantly higher than those of SLE without APS and healthy individuals. Further, antibody levels of APS patients with arterial thrombosis were significantly higher than those patients with venous thrombosis and pregnancy morbidity. Thus, oxidation of LDL leads the complex formation with beta2GPI in SLE and APS patients. In contrast, anti-beta2GPI/oxLig-1 autoantibodies were generated only in APS and were strongly associated with arterial thrombosis. These results suggest that autoantibodies against beta2GPI/oxLDL complexes are etiologically important in the development of atherosclerosis in APS.