Urine samples from patients with systemic lupus erythematosus (SLE) (n = 80), patients with rheumatoid arthritis (RA) (n = 21), and healthy controls (n = 36) were analyzed by radio-immunoassay (RIA) for anti-RNA polymerase I (RPI) antibodies. Significant levels of anti-RPI antibodies were detected in the urine of 46% of the patients with SLE but in only 19% of the patients with RA and in no sample from healthy individuals. The presence of anti-RPI antibodies in the urine was confirmed by demonstrating that IgG purified from the urine of patients with SLE was capable of inhibiting the transcriptional activity of RPI in vitro. If the quantity of anti-RPI antibodies excreted is related to disease activity, analysis of urine for these antibodies may be a useful alternative for the purpose of monitoring the progression of disease in individuals with SLE because of the ease by which the sample can be collected.