 In this prospective longitudinal study, we will test the association of several genetic markers with the severity of AS. Approximately 700 patients will be included. Measures of AS severity will be patient-reported pain and stiffness, functional disability, patient and physician global assessments, joint counts, number of tender entheses, spinal mobility, and laboratory measures of inflammation. These measures will be assessed every 6 months for 5 years. We will also evaluate new laboratory tests as measures of the activity of AS. 