A Two-Part 26-Week Study of Etoricoxib as Treatment for Ankylosing Spondylitis (AS) (MK-0663-108)

Inclusion Criteria: * Has a definite diagnosis of Ankylosing Spondylitis (AS) per Modified New York Criteria made at least 6 months prior to screening * Has a history of positive therapeutic benefit with non-steroidal anti-inflammatory drugs (NSAIDs) and regular use of NSAIDS for past 30 days * Has a score on Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) Question 2 at screening visit that is \<77 mm * Must demonstrate sufficient "flare" or worsening of AS pain * Is in general good health (other than AS) * Has had approved non-study antirheumatic therapy that has been at stable dosing AND is not anticipated to undergo a change within the first 6 weeks of the protocol Exclusion Criteria: * Has inflammatory arthritis (eg, rheumatoid arthritis, psoriatic arthritis, crystal-induced arthritis, spondyloarthropathy, diffuse idiopathic skeletal hyperostosis \[DISH\]), polymyalgia rheumatica, a history of septic arthritis or intra-articular fracture of the study joint, Wilson's disease, hemachromatosis, ochronosis, or primary osteochondromatosis * Has acute peripheral articular disease (onset within 4 weeks prior to screening) of an active (painful or swollen) peripheral arthritis * Has a history of gastric or biliary surgery, or small intestine surgery that causes clinical malabsorption * Has had an active peptic (gastric or duodenal) ulcer or history of inflammatory bowel disease * Has undergone coronary artery bypass graft surgery (CABG), angioplasty, or has a cerebrovascular accident or transient ischemic attack within the past 6 months or has active ischemic heart disease, cerebrovascular disease, or peripheral vascular disease * Has Class II-IV congestive heart failure * Has uncontrolled hypertension * Has a history of neoplastic disease, except adequately treated basal cell carcinoma or carcinoma in situ of the cervix, or malignancies that have been successfully treated ≥ 5 years prior to screening * Has history of leukemia, lymphoma, malignant melanoma, and myeloproliferative disease * Allergy to etoricoxib or naproxen, or history of a significant clinical or laboratory adverse experience associated with etoricoxib or naproxen * Has a history or family history of an inherited or acquired bleeding disorder * Is considered morbidly obese and demonstrates significant health problems stemming from obesity, which would confound study participation or interpretation of study results * Is pregnant, breast-feeding, or expecting to conceive during the study * Has a clinical diagnosis of hepatic insufficiency defined as Child-Pugh score ≥ 5