OBJECTIVE: Treatment of ankylosing spondylitis (AS) with infliximab, an anti-tumor necrosis factor alpha monoclonal antibody, was shown to be efficacious in patients with active disease during a 3-month treatment period. The purpose of this study was to evaluate the efficacy and safety of infliximab treatment of AS for a 1-year period. METHODS: This study was an open, observational, extension study of a 3-month, randomized, placebo-controlled trial. All patients who had tolerated infliximab (infliximab/infliximab group) or placebo (placebo/infliximab 12-week crossover group) therapy for 3 months entered the open extension trial (n = 65). Infliximab was administered at a dosage of 5 mg/kg every 6 weeks after the induction phase (weeks 0, 2, and 6). The primary end point was a 50% improvement in the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI). RESULTS: At week 54, a total of 54 of the 69 patients (78%) continued to take infliximab. The intent-to-treat primary efficacy analysis at week 54 showed that 47% of patients in the infliximab/infliximab group (95% confidence interval 31-63) and 51% of the patients in the placebo/infliximab group (95% confidence interval 36-67) achieved 50% improvement in BASDAI scores. In the analysis of those who completed the study, the mean BASDAI scores improved between weeks 0 and 54 in both treatment groups: from 6.6 to 2.4 in the infliximab/infliximab group and from 6.3 to 2.6 in the placebo/infliximab group. The dosage of nonsteroidal antiinflammatory drugs was reduced in approximately 70% of the patients. There were significant improvements in measures of functioning, metrologic parameters, and quality of life. Between weeks 12 and 54, a total of 4 patients had serious adverse events that were possibly related to infliximab and resulted in their discontinuing the study. CONCLUSION: Infliximab therapy in AS patients resulted in a rapid and significant improvement in BASDAI scores (>50% improvement) and a durable response for 1 year. The safety profile of infliximab in AS was comparable to that observed in the postmarketing experience for the approved indications.