Gastrointestinal inflammation or infection can be associated with various forms of arthritis, such as, acute reactive arthritis triggered by enteritis due to gram-negative bacteria or ankylosing spondylitis and peripheral arthritis in relation to Crohn's disease and ulcerative colitis. Using colonoscopy, we have found a high prevalence of clinically silent inflammatory lesions in 38 patients (24 males and 14 females) affected by undifferentiated spondyloarthropathies (SpA). Microscopic inflammatory lesions were present in all the patients. Three patterns of nonspecific chronic inflammatory alterations were observed. No difference was noted between patients taking or not taking nonsteroidal anti-inflammatory drugs. Direct immunofluorescence demonstrated the presence of IgG, IgA, IgM, C3, C4 and fibrinogen in 75% of the specimens examined. The finding of chronic inflammatory gut lesions hypothesizes that a local activation of the immune system depending on the persistence of intestinal microbial antigens or toxins, due to impaired elimination or increased exposition, may have a part in the pathogenesis of SpA.