OBJECTIVES: To estimate the annual and 5-year budgetary impact of including natalizumab in the Brazilian public health care system drug formulary for the treatment of relapsing remitting multiple sclerosis (MS). METHODS: Brazilian public health care system perspective was adopted. The baseline scenario comprised four treatment options available in Brazil in the year 2009, and its corresponding market shares: 1) interferon beta 1A intramuscular (23%); 2) interferon beta 1A subcutaneous (32%); 3) interferon beta 1B (23%); and (4) glatiramer acetate (22%). Data were retrieved from the Brazilian health system database. A claims data based approach was used to estimate the baseline population size, which was estimated to be 6935 patients. In addition to the base case scenario, three alternative scenarios were created to estimate the budgetary impact of including natalizumab to the drug formulary at 3 possible market shares: 3%, 10% or 22%. Costs for currently available treatments comprised average purchase prices paid by the Brazilian government in 2009. The adopted natalizumab dose cost was R$,652 (€1660), obtained from public data issued by the Brazilian’s regulatory health agency (ANVISA). Costs were adjusted for 4% annual inflation. No discounts were considered. Sensitivity analysis was performed. RESULTS: Base case costs for treatment of MS within a 5-year period were R$1280,698,041 (€582,135,473). Relative to this value, at 3% market share, the inclusion of natalizumab produced a 6% (R$78,858,437/€35,844,744) increase in expenditures. At 10% market share, the corresponding expenditures increased 9% (R$108,902,983/€49,501,356). At 22% market share, there was an increase of 13% in total costs with MS treatment (R$161,903,806/€73,592,639). CONCLUSIONS: Entry of natalizumab into the market is likely to result in a significant increase in MS treatment costs for the Brazilian health care system. However, this must be weighted against the need for new treatment options as well as potential savings from better disease control rates.