Peripheral involvement of the joints, including pauciarticular, asymmetrical, transitory and migrating synovitis and enthesiopathy, is observed in 10-20% of affected inflammatory bowel disease patients. Recurrence is common and frequently coincides with a flare-up of intestinal disease. The true prevalence of axial involvement is less well established. Sacroiliitis is a hallmark of spondylitis, but is under-reported due to its insidious onset and sometimes asymptomatic nature. Radiographic evidence of sacroiliitis is present in about 20-25% of patients. Ankylosing spondylitis, as defined by the Rome criteria, is present in 3-10% of inflammatory bowel disease patients, and is thought to have a different genetic predisposition in these patients compared with 'classic' ankylosing spondylitis: whereas the human leucocyte antigen B27 phenotype is present in 90% of patients with 'classic' ankylosing spondylitis, the prevalence decreases to only 30% in patients with ankylosing spondylitis secondary to Crohn's disease. Polymorphisms involving CARD15 appear to be a possible genetic trigger: 78% of patients with Crohn's disease and symptomatic or asymptomatic sacroiliitis carry at least one mutation, compared with only 48% of control Crohn's disease patients. Moreover, in other forms of spondyloarthropathy, a similar association has been reported: 42% of patients with spondyloarthropathy and associated asymptomatic chronic gut inflammation, who are considered likely to develop Crohn's disease and ankylosing spondylitis, are carriers of at least one CARD15 mutation, compared with only 7% of patients with normal histology. In addition to genetic markers, clinical features support the relationship between gut and joint pathophysiology. In cases of spondyloarthropathy, a very rapid, substantial and sustained improvement in symptoms has been reported following treatment with infliximab, suggesting an essential role for tumour necrosis factor-alpha in spondyloarthropathy, similar to that observed in Crohn's disease.