The main objectives of the present study were to determine the prevalence of IgG and IgM anticardiolipin antibody (aCL) isotypes in systemic lupus erythematosus (SLE) in order to analyze their possible association with the clinical manifestations of the disease. Clinical features of 64 consecutive and unselected SLE patients were prospectively studied. Sera from the same patients taken during each clinical manifestation were tested for the presence of aCL. The prevalence of aCL was 43.75% for the IgG isotype and 9.4% for the IgM isotype. A strong linkage between the presence of these antibodies and the occurrence of both thrombosis and abortions was found: a weaker association with neurological events and thrombocytopenia was also demonstrated. The titre of aCL appeared to be linked with the probability of having the clinical manifestations associated with these autoantibodies. Our results suggest that thrombosis and abortion, and possibly thrombocytopenia and central nervous system involvement, may be associated with the presence of aCL at the time when these clinical events develop in SLE patients.