Ileocolonoscopy on patients with HLA-B27 related disease, reactive arthritis and ankylosing spondylitis (AS) with peripheral arthritis, revealed the presence of inflammatory lesions in biopsy specimens from the ileum in 71 and 60% of the cases, respectively. A 2nd ileocolonoscopy was performed on 14 patients, 4 to 22 months after the first. In 6 of the 7 patients in articular clinical remission, the inflammatory gut lesions had disappeared. The gut biopsies of the 7 patients with active joint disease continued to show inflammatory lesions. One of these patients underwent a 3rd ileocolonoscopy after having gone into clinical remission, possibly secondary to treatment with sulfasalazine; the histology was completely normal. The strong relationship between clinical articular inflammation and gut inflammation seems to support the hypothesis that in B27 related diseases, repetitive exogenous triggering causing transgression of oral tolerance by increased gut permeability, provokes and maintains the joint inflammation.