OBJECTIVES: Limited information exists on real-world use of biologic agents in ankylosing spondylitis (AS). This study evaluated the treatment patterns and costs of biologic therapy and disease-modifying antirheumatic drugs (DMARDs) in AS patients.  METHODS: MarketScan claims databases were used to identify biologics treatment-naïve AS patients (≥18 years of age with no biologics in the past 6 months) who initiated biologic treatment between 10/1/2009 and 09/30/2010. Frequency of biologic switching, duration, modification of treatment, and medical and pharmacy drug costs for each line of biologic therapy was analyzed during the 3-year follow-up. RESULTS: A cohort of 339 AS patients was identified.  First-line biologics were: etanercept (n=140), adalimumab (n=137), infliximab (n=41), golimumab (n=18) and certolizumab pegol (n=3).  Approximately two-thirds of the patients (64.6%) discontinued their first-line biologic therapy and did not restart any biological therapy, 23.6% of patients stayed on their first-line, 8.8% switched to second-line and 2.9% to third-line therapies. Patients who switched to a second- or third-line biologic treatment remained on their first-line agent for less time (336, 325-days, respectively) than those who did not switch (502-days). Most common treatment modifications in first-line were DMARD add-on (6.5%) and removal (5.6%). Time to first treatment-modification was shorter for those who switched to second- (88 days) and third-line therapy (6-days) vs. those who remained on first-line (160-days). Monthly-per-member medical costs were greatest for non-switchers ($363) than for patients who received second-($249) or third-line ($113) treatment.  Monthly-per-member overall pharmacy drug costs were similar for patients with first-($1,908), second-($1,955) or third-line and above ($1,890) therapy. CONCLUSIONS: Sixty five percent of AS patients discontinued their first-line agent during the follow-up period. Those who switched biologics had more modifications to their treatment.  Patients who switched to second- or third-line biologics had lower medical costs; however, their pharmacy costs were similar to those who continued their first-line.