The effect and tolerance of proquazone and indometbacin in patients suffering from ankylosing spondylitis were compared in two separate 3-week clinical, double-blind, randomized studies on a total of 60 patients. In the first study 16 patients were treated with proquazone and 14 patients with indomethacin, and in the second study 15 patients were treated in each group. Both drugs were found to have an evident, equally strong effect on pain and functional capacity in daily doses of 900 mg and 75 mg respectively. Severe side effects were not noted with either one of the drugs. Patients with previously known intolerance to indomethacin were excluded from the studies. Gastrointestinal disturbances seem to be the most commonly appearing side effect in patients treated with proquazone.