We evaluated the clinical relevance of 6 antiphospholipid antibodies including cardiolipin and their IgG, IgM and IgA isotypes in 92 patients with systemic lupus erythematosus (SLE). Antiphospholipid antibodies generally had significant associations with thrombocytopenia and a history of false-positive syphilis serologies. In 4 of 6 antiphospholipid antibodies, an inverse association with renal disease was observed. Antiphospholipid antibodies may moderate or protect against renal disease, or this may reflect the high doses of corticosteroids and cytotoxic drugs received by this group. Further studies are needed to determine how many antibody families cause these activities and to elucidate whether certain SLE subgroups possess differing specificities for each of the phospholipids.