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OBJECTIVE: To estimate the cost-effectiveness of the treatment of ankylosing spondylitis (AS) with infliximab (Remicade) in Canada over the long term, with both international and Canadian treatment regimens. METHODS: A previously published disease model based on functional capacity and disease activity was adapted to the Canadian setting. Current resource consumption from a cross-sectional bottom-up burden-of-illness study in 545 patients at different levels of severity of AS in 4 Canadian provinces was incorporated into the model. Cost-effectiveness estimates were based on a 3-month placebo-controlled clinical trial with 2-year open extension as well as a 4-year followup study of clinical practice in Canada. In the cost-effectiveness model, patients with insufficient response to treatment at 12 weeks (> or = 50% reduction in Bath Ankylosing Spondylitis Disease Activity Index) discontinue treatment. In view of the long disease duration, simulations over a 30-year timeframe were performed, incorporating disease progression from cohort studies and assumptions about treatment continuation beyond the clinical trial from the trial extension period. Results are presented in Canadian dollars, from the societal and healthcare payer perspectives, with both costs and effects discounted at 5%. RESULTS: Over a 30-year timeframe, with the assumption that patients' disease would remain stable while on treatment, the cost per quality-adjusted life-year (QALY) gained in the societal perspective is $37,491, using the treatment regimen in the clinical trial (5 mg/kg every 6 weeks). Using the dosing regimen of the Canadian study (75% at 3 mg/kg every 8 weeks, 15% at 3 mg/kg every 6 weeks, and 10% at 5 mg/kg every 8 weeks) the cost per QALY is dollar 10,264. Assuming that patients on treatment progress at half the rate of untreated patients, the cost-effectiveness ratios are dollar 45,121 and dollar 13,883, respectively, while the most conservative assumption that progression is the same in both arms, the ratios are dollar 54,137 and dollar 18,712, respectively. The results are sensitive to the dosing regimen adopted, the discontinuation rate, and assumptions concerning disease progression while on treatment. CONCLUSION: Our results indicate that infliximab therapy for patients with active AS would be cost-effective (ranges dollar 10,264-dollar 54,137 per QALY) in a Canadian setting.