Epidemiological studies on the spondyloarthritides have been hindered in the past by the lack of adequate classification criteria for the whole group of these diseases. Using the Amor and the European Spondyloathropathy Study Group (ESSG) criteria the total prevalence of such diseases has been found to be higher than estimated in the past. The prevalence of ankylosing spondylitis varies across populations, but closely parallels the frequency of HLA B27-associated subtypes. The lack of well established criteria for reactive arthritis and the varying expression of its clinical manifestations are the principal reasons for the under-reporting of the true prevalence and incidence of this type of spondyloarthritis. Few data exist on the prevalence and incidence of psoriatic arthritis. A recent European study on an inception cohort of patients having inflammatory bowel disease has evaluated the prevalence of spondyloarthritis using the ESSG criteria. Of the patients studied, 18% met these criteria. Undifferentiated spondyloarthritis is one of the most frequent spondyloarthritides. It also includes a number of different subtypes.