SummaryThe significance of anti‐cardiolipin antibodies (ACLA) in patients with a range of autoimmune and infective disorders was investigated in this study. Although lower levels of IgG and IgM ACLA were present in 77 of 400 patients' sera (19%), high antibody levels were found in<5% of patients. These latter patients belonged to three clinical categories: patients with connective tissue disease (CTD), infectious mononucleosis or biological false positive serology. An assay was developed to measure lupus anticoagulant (LA) activity in serum: significant LA activity was found in the CTD patients alone (in 6 of 15 tested) and all of these had high titre ACLA. Features of the antiphospholipid syndrome (APS) were present in these six patients but also in three additional CTD patients with normal LA results and high levels of ACLA. In two CTD patients with APS features, the high ACLA were of IgM isotype. These results stress the importance of measuring both ACLA and LA in an investigation of the APS: a high positive ACLA supports the diagnosis of APS, particularly in patients with autoimmune disease, whilst a high positive ACLA in association with LA activity is specific for this syndrom