Intestinal permeability was measured using cow's milk beta-lactoglobulin absorption (BLG) as a permeability marker in 14 patients with active and inactive rheumatoid arthritis (RA) under three different conditions: after a washout period, after treatment with acetylsalicylic acid (ASA) associated with disodium chromoglycate (DSCG), and with ASA only. No intolerance to cow's milk was present and serum IgE levels were in the normal range in 12 of 14 patients. IgG anti-IgE were present in 7 of 13 patients tested. When off treatment the intestinal permeability to BLG in RA patients was not increased as compared to controls, but we found a significative difference between active and inactive RA. ASA administration strongly increased BLG absorption, not prevented by DSCG pretreatment. In normal controls treated with a single dose of ASA we obtained similar results. Our results suggest that prolonged treatment with nonsteroidal anti-inflammatory drugs induces an increase of food antigen absorption, apparently not related to anaphylaxis mediator release, with possible clinical effects.