Seronegative spondylarthropathies include ankylosing spondylitis, Reiter's syndrome and reactive arthritis, psoriatic arthritis, arthritis associated with ulcerative colitis and Crohn's disease, plus other forms which do not meet the criteria for definite categories and are called undifferentiated. Recently two sets of classification criteria have been proposed for the entire group including undifferentiated forms: the European Spondylarthropathy Study Group and the Amor criteria. The prevalence of spondylarthropathies is directly correlated with the prevalence of the HLA-B27 antigen in the population. The highest prevalence of ankylosing spondylitis (4.5%) has been found in Canadian Haida Indians, where 50% of the population is B27 positive. Among Europeans the frequency of the B27 antigen in the general population ranges from 3 to 13% and the prevalence of ankylosing spondylitis is estimated to be 0.1-0.23%. Seronegative spondylarthropathies have common clinical and radiologic manifestations: inflammatory spinal pain, sacroiliitis, chest wall pain, peripheral arthritis, peripheral enthesitis, dactylitis, lesions of the lung apices, conjunctivitis, uveitis and aortic incompetence together with conduction disturbances. All of these may also occur in isolation.