The purpose of the study is to evaluate the efficacy and tolerability of two doses of etoricoxib compared to naproxen in the treatment of ankylosing spondylitis (AS). The primary objectives are to evaluate the improvement in Spinal Pain Intensity over 6 weeks of treatment with etoricoxib 90 mg or 60 mg compared to naproxen; and to evaluate the improvement in Spinal Pain Intensity over 6 weeks of treatment with etoricoxib 90 mg compared with etoricoxib 60 mg. Additionally the added benefit of increasing the dose of etoricoxib from 60 mg to 90 mg will be assessed in the second part of the study. The primary hypothesis is that the improvement in Spinal Pain Intensity visual analog scale (VAS) as measured by the time-weighted average (TWA) change from baseline over 6 weeks of treatment in Part I for etoricoxib 90 mg or 60 mg once daily is not inferior to naproxen 1000 mg.