 The susceptibility to ankylosing spondylitis AS is largely genetically determined. Recent studies suggest that the severity of AS is also influenced to a large extent by genetic factors. The goal of this study is to identify genes that influence the severity of AS. We hypothesize that genetic markers of susceptibility, including human leukocyte antigen HLA polymorphisms, are among the genes that also influence the severity of AS. Other genes that could potentially influence the severity of AS include those that affect the severity of joint inflammation and those that influence bone formation. 