Antiphospholipid antibodies (aPL) have been linked to various clinical manifestations in systemic lupus erythematosus (SLE), mainly thrombosis, repeated abortions and thrombocytopenia. Despite the large number of studies which have been published in the recent years, there is still some debate on this matter, and no firm conclusion has been reached as yet. Among the various aPL, anticardiolipin antibodies (aCL) have received more attention, mostly because they can be easily detected by means of immunoenzymatic assays. The main objective of the present study was to determine the prevalence of IgG and IgM aCL isotypes in SLE in order to compare their possible association with the clinical manifestations of the disease. Clinical features of 40 consecutive and unselected SLE patients (35 female and 5 male) were prospectively studied. Sera from the same patients were tested for the presence of aCL, using a standardised ELISA assay, and the presence of aCL was correlated with the various clinical events. Results showed that the prevalence of aCL was 42.5% for the IgG isotype and 10% for the IgM isotype. Regarding the clinical associations of aCL, we found a strong linkage between the presence of these antibodies and the occurrence of both thrombosis and abortions; a weaker association with neurological events was also demonstrated. These results, if confirmed on larger series, suggest that aCL should be searched in patients with SLE in order to identify those who are at greater risk of developing some severe clinical problems and could benefit by prophylactic treatment.