The prevalence of anticardiolipin antibodies in active systemic lupus erythematosus (SLE) was compared with that in inactive SLE and other rheumatic and non-rheumatic diseases to determine the value of these autoantibodies in monitoring rheumatic diseases. Pairs of IgG- and IgM-aCL were measured by ELISA in 173 consecutive hospitalised patients, including 141 with rheumatic diseases (18 active SLE, 21 inactive SLE, 19 rheumatoid arthritis, 13 reactive arthritis, 7 other spondyloarthropathies, 16 vasculitis, 47 other autoimmune diseases) and 32 non-rheumatic controls. A further 101 aCL pairs were determined during follow-up in 19 patients with SLE. Serum concentrations were analysed with respect to SLE activity and compared between the different patient groups. IgG- and IgM-aCL levels in excess of 10 GPL and 9 MPL respectively were considered positive. 30.6% of all patients (53/173) were found to be positive for IgG-aCL, as against only 9.8% (17/173) for IgM-aCL. IgG-aCL serum levels in active SLE differed significantly from all other groups, including inactive SLE (all p < 0.005). Median IgM-aCL levels were below the cut off point in all groups, although measurable values were obtained almost exclusively in active SLE and RA. In this study IgM-aCL measurement was of less value in monitoring rheumatic diseases. IgG-aCL positivity in SLE was associated with a significantly higher odds ratio (OR) for active disease (OR 16.0, 95% confidence interval: 2.8-90.0). The results show that disease activity in SLE was accompanied by significantly increased IgG-aCL, whereas no elevation was found in other diseases. This parameter may therefore be useful in monitoring SLE activity.