Ankylosing spondylitis (AS) is a type of arthritis which is the most well-known of a group of disorders called the seronegative spondyloarthropathies. AS mostly affects the joints of the spine and the pelvic sacroiliac joints. Eye and bowel problems may also occur.

Back pain and stiffness of the spine is a characteristic symptom of AS. Between 0.1% and 1.8% of people are affected. More than 90% of those affected have on testing, a specific human white cell molecule known as the HLA-B27 antigen. This antigen can be found when there is no evidence for ankylosing spondylitis.

The prevalence of HLA-B27 varies between populations— from 50% in Haida Indians to nil in Australian Aborigines. In the UK general population, it is about 8%. HLA-B27 is rare in the American black population whereas Eskimo populations carry it much more frequently than Western Europeans, with prevalence rates of 25% or more.