Serum levels of immunoglobulins (Ig) A, G, and M anti-cardiolipin (aCL) antibodies were determined by enzyme-linked immunosorbent assay in a group of selected systemic lupus erythematosus female patients. Patients were divided into three groups based on their clinical history of thrombosis with or without thrombocytopenia (group I), thrombocytopenia alone (group II), and neither of these (group III). After the aCL antibody levels were determined, the patients' obstetric histories of pregnancies and abortions were reviewed. A high incidence of one or more abortions was seen only in group I patients. A high prevalence of elevated levels of IgA and IgG (but not IgM) aCL antibodies was observed in group I patients. However, among the patients in group II, only the levels of IgA aCL antibodies were increased. In both groups, the addition of the IgA aCL determination--to the classical IgG and IgM aCL assays--increased the prevalence of positive reactors in 31.6%. These results indicate that high levels of IgA aCL antibodies correlated better with thrombocytopenia than either IgG or IgM. Also, the importance of including the determination of IgA aCL antibodies to assess properly the risk of thrombosis, thrombocytopenia, and recurrent abortions in systemic lupus erythematosus patients is demonstrated.