Ileocolonoscopy and microscopic examination of ileum biopsies were performed on 35 patients with reactive arthritis, with asymmetrical pauciarticular arthritis and enthesopathies. Ileocolonoscopy was also performed on 26 patients with ankylosing spondylitis (AS) and on 19 control patients with rheumatoid arthritis, juvenile chronic arthritis, systemic lupus erythematosus and psoriatic arthritis. In the reactive group, ileocolonoscopy showed macroscopic inflammation in 16 cases and abnormal microscopic examination in all but 2 cases, even in patients without gastrointestinal disorders. In the 2 patients with sexually acquired disease, the gut was normal. In the AS group, inflammation was observed in the B27 negative and positive patients with peripheral joint involvement. Occasionally, ileal signs were seen in the HLA-B27 positive patients without peripheral joint involvement. None of the controls showed signs of gut inflammation. Ilecolonoscopy may be of value in detecting subclinical forms of bowel inflammation.