Ileocolonoscopy with biopsy of caecum, ileocaecal valve and terminal ileum were performed on 232 patients with seronegative spondylarthropathies and on 65 control patients. Inflammatory gut lesions were found in 65% of the patients with reactive arthritis (ReA) and in 57% of the patients with ankylosing spondylitis (AS), especially in those with peripheral arthritis. The controls had a normal gut. This finding would suggest that exogenous factors causing inflammation of the gut lead to a disturbed permeability of the gut wall or to a deficient local immunological defence mechanism permitting antigens to enter the circulation, inducing the joint and tendon inflammation. Support for this hypothesis was provided by the results of a repeat ileocolonoscopy, disclosing a strong association between the presence of gut inflammation on biopsy and the persistence of joint inflammation. Patients presenting some of the histological lesions found on biopsy (especially active chronic lesions) and patients with proven Crohn's disease were found to share a genetic marker (HLA-BW62). This would suggest that some of the patients with seronegative spondylarthropathies suffer from a subclinical form of Crohn's disease of which the joint symptoms are the unique clinical manifestation.