The efficacy and tolerance of 1200 mg/day oxaprozin and 100 mg/day diclofenac sodium were compared in 40 patients with ankylosing spondylitis in a 6-week open study. Overall improvement was seen in the patients in both treatment groups. Oxaprozin-treated patients showed significant improvement in spontaneous pain of the vertebral spine and in morning stiffness after 6 weeks' treatment. There were no statistically significant differences between the treatment groups. Therapy was discontinued in 10 patients; five treated with oxaprozin (three because of intolerance and two because of worsening of symptoms) and five taking diclofenac sodium (four because of intolerance and one because of worsening of symptoms). Five (25%) oxaprozin-treated patients and six (30%) diclofenac sodium-treated patients had side-effects, with gastro-intestinal disturbances and dizziness reported most frequently. There were no statistically significant differences between the groups in the frequency of side-effects. These results indicate that oxaprozin is a promising therapeutic agent for ankylosing spondylitis.