To establish the prevalence of peripheral arthritis, radiographic sacroiliitis, and ankylosing spondylitis in patients with inflammatory bowel disease, 58 consecutive patients suffering from ulcerative colitis (UC) and 51 with Crohn's disease (CD) underwent a detailed rheumatological examination. In addition, all patients were screened for the presence of the antigen HLA B27. Peripheral arthritis was found in 14 (8 UC, 6 CD) patients (12.8%); radiographic sacroiliitis was diagnosed in 11 (5 UC, 6 CD) (10.1%), of whom 10 were asymptomatic; and ankylosing spondylitis was diagnosed in 2 UC and 2 CD patients (3.7%). 18.9% of the UC and 3.9% of the CD patients were HLA B27 positive. One of the 11 patients with radiographic sacroiliitis and 2 of the 4 with ankylosing spondylitis had the HLA B27 antigen. Peripheral arthritis, radiographic sacroiliitis, and ankylosing spondylitis are apparently frequent manifestations in patients suffering from inflammatory bowel disease. Asymptomatic radiographic sacroiliitis in these patients appears to differ from idiopathic ankylosing spondylitis, both clinically and genetically. Evaluation of subjective rheumatological complaints, necessary for a confident diagnosis of ankylosing spondylitis, according to the New York criteria is difficult during a flare-up of the inflammatory bowel process, as was shown in 4 CD cases with marked limitation of lumbovertebral function and chest expansion, but no radiological abnormalities of the SI joints.