<qa>  How many patients will have AS for at least 20 years?In this cross-sectional study, we will test the association of HLA markers and polymorphisms in several genes involved in the regulation of inflammation and bone formation with the severity of AS. Approximately 500 patients will be enrolled. Each patient will have had AS for at least 20 years. Measures of AS severity will be the extent of spinal fusion as measured radiographically, functional disability, time to permanent work disability, and need for total hip arthroplasty. 