Intestinal permeability in controls and in patients with active rheumatoid arthritis was assessed by measuring the radioactivity in 24-h urine collections after oral administration of 51Cr-EDTA. Intestinal permeability in the patients was found to be significantly increased in comparison to controls. Of the patients, 6/7 (86%) not taking any prescribed nonsteroidal anti-inflammatory drug (NSAID) at the time of study had normal results, whereas 23/29 patients (79%) taking prescribed NSAIDs displayed increased intestinal permeability. It has not been determined beyond doubt whether this finding is due to disease process or therapy with oral NSAIDs.