Background This study evaluated two doses of etoricoxib (60 and 90 mg) vs. naproxen 1000 mg in subjects with ankylosing spondylitis (AS). Methods This was a 2-part, double-blind, active comparator-controlled non-inferiority study in subjects ≥18 years of age with AS. In Part I, subjects were randomized to naproxen 1000 mg; etoricoxib 60 mg, and 90 mg. In Part II, naproxen and etoricoxib 90 mg subjects continued on the same treatment; subjects on etoricoxib 60 mg either continued on 60 mg or escalated to 90 mg. Part I (6 weeks) assessed the efficacy of A) etoricoxib 60 mg vs. naproxen and B) 90 mg vs. naproxen according to the time-weighted average change from baseline in Spinal Pain Intensity (SPI; 0–100 mm VAS) (primary endpoint). The non- inferiority margin was set at 8 mm for SPI. In Part II (20 weeks) we evaluated the potential benefit of increasing from 60 to 90 mg (predefined minimum clinically important difference = 6 mm in SPI) for inadequate responders (<50 % improvement from baseline in SPI) onetoricoxib 60 mg in Part I. Results In total, 1015 subjects were randomized to receive etoricoxib 60 mg (N = 702), etoricoxib 90 mg (N = 156), and naproxen 1000 mg (N = 157); 70.9 % were male and the mean age was 45.2 years. There were 919 subjects who completed Part I and all continued to Part II. In Part I, SPI change was non-inferior for both etoricoxib doses vs. naproxen. In both Part I and II, the incidence of adverse events (AEs), drug-related AEs, and serious adverse events (SAEs) were similar between the 3 treatment groups. Conclusion Both doses of etoricoxib were non- inferior to naproxen. All treatments were well tolerated. Etoricoxib 60 and 90 mg effectively control pain in patients with AS, with 60 mg once daily as the lowest effective dose for most patients. Trial registration Clinical Trials Registry # NCT01208207. Registered on 22 September 2010.