In a multicentre study anticardiolipin antibodies of the IgG and IgM isotypes were measured by a solid phase enzyme immunoassay in 368 patients with systemic lupus erythematosus (SLE) who were not selected on the basis of features of antiphospholipid syndrome. Clinical and laboratory associations of increased levels of anticardiolipin antibodies were evaluated. IgG and IgM antibodies to cardiolipin were documented in 224 (60.9%) and 128 (34.8%) patients, respectively. Regarding the symptoms of antiphospholipid syndrome, elevated amounts of anticardiolipin IgG were significantly associated with spontaneous abortion (P < 0.001), thrombocytopenia (P < 0.01), livedo reticularis (P < 0.01) and a positive direct Coombs test (P < 0.05), but not with thrombosis or central nervous system diseases such as epilepsy and psychosis. IgM antibodies to cardiolipin were associated with a positive direct Coombs test (P < 0.01), but with no other symptom of antiphospholipid syndrome. The predictive values of anticardiolipin antibody determinations in unselected SLE patients were poor for all features of antiphospholipid syndrome because of high proportions of false-positive and false-negative results. As for other manifestations of SLE, positive correlations between raised antibodies to double-stranded DNA and the occurrence of anticardiolipin antibodies of the IgG isotype were observed, and anticardiolipin IgM was negatively associated with nephritis.