The prevalence of anticardiolipin antibodies (ACA) was determined using an Elisa method in sera of 60 patients with autoimmune diseases. ACA were correlated with clinical manifestations of the antiphospholipid syndrome and other serological markers of autoimmunity. Sixty three percent of sera had ACA IgG (+) and 50% ACA IgM (+). Nine patients, five of them with systemic lupus erythematosus, had a history of arterial or venous thrombosis and all had positive ACA (IgG (+) in 7 and IgM (+) in 2). In patients with rheumatoid arthritis, no association was found between the presence of ACA and thrombosis. Patients with a history of spontaneous abortion had non significantly higher levels of ACA. There was a significant correlation between ACA levels and rheumatoid factor (ACA IgG r = 0.374, ACA IgM r = 0.676), no other association was found between ACA and other autoantibodies. It is concluded that ACA are found frequently in patients with autoimmune diseases and its clinical significance may be different in patients with lupus than in patients with rheumatoid arthritis or other connective tissue diseases.