In this study, we evaluated the prevalence and association with thrombosis and/or thrombocytopenia of IgG and IgM antibodies to cardiolipin (aCL), phosphatidic acid (aPA), phosphatidylinositol (aPI), phosphatidylserine, and beta(2)-glycoprotein I (abeta(2)-GPI) in systemic lupus erythematosus (SLE). Sera were obtained from 87 patients affected by SLE (77 of the 87 patients were females), 41 of them with a history of arterial and/or venous thrombosis. Antiphospholipid antibodies and abeta(2)-GPI were evaluated by enzyme-linked immunosorbent assay. IgG-aCL, IgG-aPA, IgG-aPI, IgG-aPS, and IgG-abeta(2)-GPI were found in 53%, 37%, 32%, 38%, and 24% of patients, respectively. IgM-aCL, IgM-aPA, IgM-aPI, IgM-aPS, and IgM-abeta(2)-GPI were detected in 15%, 17%, 18%, 14%, and 16%, respectively. With respect to antibody titer, IgG-aCL strongly correlated with all other antiphospholipid antibodies and abeta(2)-GPI of IgG isotype. Thrombosis was significantly associated with IgG-aPA (p = 0.044), IgG-aPI (p = 0.038), IgG-aPS (p = 0.026), IgG-abeta(2)-GPI, IgM-aPA (p = 0.044), IgM-aPI (p = 0.024), and IgM-aPS (p = 0.01), irrespective of antibody titer, whereas IgG-aCL were associated with thrombosis and thrombocytopenia when taken at medium-high titer (p = 0.009 and p = 0.046, respectively). Our results confirm that, besides aCL and abeta(2)-GPI, other antibodies to negatively-charged phospholipids are present in a large percentage of patients with SLE. However, it remains doubtful whether these other antiphospolipid antibodies actually represent an important parameter predictive of thrombosis and thrombocytopenia in SLE.