A prospective study of IgG and IgM isotypes of anticardiolipin antibodies (aCL) was performed in a series of 167 patients with various autoimmune diseases, including rheumatic and nonrheumatic disorders, and in a group of 100 healthy blood donors. The IgG aCL serum was regarded as positive if a binding index (BI) greater than 2.85 (3.77 SD) was detected and a BI greater than 4.07 (3.90 SD) was defined as positive for IgM aCL. Forty patients (24%) were found to be positive for IgG and/or IgM aCL. IgG aCL were detected in 23% of patients with systemic lupus erythematosus (SLE), in 9% with idiopathic thrombocytopenic purpura, in 7% with progressive systemic sclerosis, and in 6% with dermatomyositis-polymyositis. IgM aCL were present in 43% patients with primary biliary cirrhosis, in 33% with rheumatoid arthritis, in 22% with SLE, and in 8% with giant-cell arteritis. IgG aCL were found to have a significant association with thrombosis and thrombocytopenia, and IgM and aCL with haemolytic anaemia and neutropenia, in SLE but not in the other autoimmune diseases. The identification of these differences in the aCL isotype associations, depending on the autoimmune disorder, may improve the clinical usefulness of these tests.