Ankylosing spondylitis affects about 1% of the population. In the past, evaluation of therapy in the management of this disease has been hampered by the lack of availability of objective criteria for following the condition. By using recently developed measurements of spinal mobility and other variables we have compared sulindac, a recently introduced nonsteroidal antiinflammatory drug, and indomethacin in a double-blind six-month parallel study of 30 patients. Sulindac and indomethacin have comparable efficacy and tolerance. Advantages of sulindac include a twice-a-day dose regimen.