The suggestion that the intestinal mucosa may be abnormally permeable and thus a site of antigen absorption in rheumatoid arthritis was tested by a 51Cr EDTA intestinal permeability test. Twelve patients with rheumatoid arthritis untreated by non-steroidal anti-inflammatory drugs (NSAIDs) had normal test results, while 12 NSAID-treated patients had increased intestinal permeability. Ten volunteers ingested aspirin, ibuprofen and indomethacin 8 and 1 hours before the study. The increased intestinal permeability was proportional to drug potency to inhibit cyclo-oxygenase. Intestinal permeability also increased following an indomethacin suppository, which suggests that the effect is systemically mediated. 111Indium leucocyte scintigrams and faecal collection showed no evidence of intestinal inflammation in 9 patients untreated by NSAIDs. Twenty-nine of 53 NSAID-treated patients showed abnormal localisation of 111indium in the right iliac fossa at 20 hours, and 32 of 49 patients had increased faecal excretion of 111indium. A 99mTc-porphyrin scan suggested that the main site of NSAID-induced intestinal inflammation was the small bowel. NSAIDs are thus shown to disrupt intestinal integrity and long term treatment leads to inflammation of the small intestine.