OBJECTIVES: We described the cost of the drug therapy for ankylosing spondylitis in the State of Minas Gerais, Brazil. METHODS: We analysed data from the Outpatient Information System during March 2010 and February 2011 of the Brazilian Public Health System (SUS). We applied the probabilistic record linkage to match registers of the same patient and performed the analysis under the SUS’s perspective. We included patients over 18 years-old with diagnosis of ankylosing spondylitis who had begun the drug therapy after March 2010. We classified the patients in groups according to the initial therapy in anti-TNF agent users (infliximab, etanercept or adalimumab) or disease-modifying anti-rheumatic drugs-DMARD (the reference group). The cost was expressed in U.S. dollars. The analyses were performed with database manager system mysql and SPSS®19. RESULTS: We indentified 236 ankylosing spondylitis patients and 979 pharmacy claims. Mean age was 41.1 years (standard deviation 12.2) and most patients (70.0%) were men. Approximately 18% of patients withdrew the treatment. The total treatment expenditure in the study period was U$ 351,504.05; the cost with adalimumab represented 52.3% of this value. The estimated annual cost was U$ 117.53 for those who started sulfasalazine-DMARD treatment and ranged between U$ 3,251.08 and U$ 6,374.51 for anti-TNF drugs. The median individual cost was U$ 2,553.90 for patients who initiated infliximab, U$ 2,516.44 for adalimumab, U$ 1,406.08 for etanercept and U$ 49.08 for sulfasalazine-DMARD therapy. CONCLUSIONS: The cost of the anti-TNF agent treatment for ankylosing spondylitis is elevated. Therefore, the management of the public health system faces a challenge to offer an efficient and effectiveness patient care considering the limited public resources and the high demand for new health technologies.