The randomized, double-blind trial was carried out with 50 patients suffering from ankylosing spondylarthritis and belonging all to type B27 in the HLA system. Tolfenamic acid (600 mg daily) was effective as judged by subjective and numerous objective parameters and was preferable to indomethacin (75 mg daily). Indomethacin caused side effects more frequently than tolfenamic acid. In the indomethacin group 16% of the patients interrupted the 6-month trial because of gastrointestinal complications. No discontinuation of treatment owing to adverse effects of tolfenamic acid occurred. Tolfenamic acid offers one new and good alternative for the medical treatment of ankylosing spondylarthritis.