The role of infection in ankylosing spondylitis is speculative and different studies by several groups have yielded conflicting results. The role of infection, however, in reactive arthritis and Reiter's syndrome is well established. The Grampian region has experienced three outbreaks of gastroenteritis, two due to Campylobacter jejuni and one due to Salmonella typhimurium. These outbreaks have allowed a study of reactive arthritis in the affected population following these infections. These studies do not allow accurate estimate of the frequency of reactive arthritis, largely because of difficulties in defining the infected population accurately. They do, however, suggest that in the population milder cases occur than in a population referred to hospital rheumatology clinics. These milder cases have a lower frequency of HLA-B27 than patients seen in a rheumatology clinic and suggest that there may be an association between HLA-B27 and the severity of reactive arthritis. An association between gastroenteritis due to Campylobacter jejuni and Salmonella typhimurium and erythema nodosum both with and without arthritis was observed. No difference was detected in the immunoglobulin responses of immunoglobulins A, G and M between patients with reactive arthritis due to Salmonella typhimurium and patients with gastroenteritis alone.