A clinical trial to assess the effect of celecoxib 200 milligrams (mg) once daily and 400 milligrams (mg) once daily compared to diclofenac three times daily in the treatment of Ankylosing Spondylitis (AS) for 12 weeks. This will be used to confirm the results of a prior 6 week trial.

Inclusion Criteria:

• Male or female, 18 to 75 years of age, inclusive.
• Clinical diagnosis of ankylosing spondylitis:
• Willing to stop existing NSAID/COX-2 inhibitor treatment for up to two weeks.
• Having given written informed consent to participate in the trial.
• Pain intensity ≥ 40 mm on the visual analog scale, worsening by at least 30 % compared to that recorded at the screening visit.
• Last administration of analgesic without anti-inflammatory activity is ≥ 8 h, the last administration of long-acting NSAIDs is ≥ 72 h

Exclusion Criteria:

• Patients with acute peripheral articular disease (defined by the onset within 4 weeks prior to visit
• Known inflammatory enteropathy (e.g. ulcerative colitis, Crohn's disease).
• Ongoing extra-articular signs (e.g. cardiac involvement).
• Current painful vertebral compression.
• Requirement to start physiotherapy, re-education or manipulation
• History of clinical gastroduodenal ulcer in the year preceding inclusion, confirmed by endoscopy; continuing gastro-intestinal bleeding.
• Cardiac failure or known renal insufficiency that could be affected by study medication, chronic or acute hepatic insufficiency, significant coagulation disorders or history of asthma.
• Current or history of malignancy (except: patients having a basal cell carcinoma or other malignancy operated on and in remission for 5 years before inclusion in the trial).
• Pregnancy, women of childbearing potential not using adequate contraceptive methods or nursing mothers.
• Subject who has evidence of alcohol or drug abuse.
• Participation in any other clinical study within 30 days prior to the screening visit.
• Any condition that would prevent the patient from entering the study, according to the investigator's judgment.
• Known hypersensitivity to celecoxib, sulphonamides or any NSAID (including salicylic acid).
• Taking the following medications: Muscle relaxants, hypnotic, anxiolytics, sedatives, tranquillizers or antidepressants (unless stable for 2 weeks before screening and continuing throughout the trial), NSAIDs or COX-2 inhibitors other than study medication, acetylsalicylic acid (ASA) > 160 mg /day, anti-coagulants, ticlopidine, lithium, anti-TNF agents or methotrexate > 15 mg/week (Note: ASA ≤ 160 mg/day for cardioprotection is permitted), Use of oral or systemic analgesic medication, except from paracetamol, within 3 days of study entry and through the study, Corticosteroids (PO/IM/IV/IA) in the 6 weeks preceding inclusion in the trial, Anti-ulcer medication including chronic (daily or almost daily) use of antacids [Note: Occasional use of antacids during the study will be permitted.]
• Change in dose of a slow-acting drug (e.g. sulfasalazin, methotrexate) in the past 60 days preceding inclusion in the trial.
• Taking paracetamol > 2000 mg/day (including during the screening period).