The increased frequency of thromboembolic events in patients with systemic lupus erythematosus (SLE) has been attributed to reduced or dysfunctional antithrombin III (At-III). We analysed At-III values, measured by three different assay techniques, in SLE patients, patients with rheumatoid arthritis, and normal and hospitalised controls. In addition, attempts were made to correlate At-III activities of SLE patients with specific clinical and serological parameters such as disease activity, renal involvement, previous thrombosis, degree of proteinuria, and serum complement concentrations. Our results failed to show a significantly reduced At-III in SLE with any method. At-III titres did not correlate with disease activity, concentrations of serum complement or albumin (both only minimally reduced in most patients), or a previous history of thrombosis. At-III deficiency does not appear to be an inherent feature of SLE, and reduced activities should only be anticipated when there are specific aetiological factors present, such as massive proteinuria, extensive hepatic disease, or active thrombosis.