OBJECTIVES: To estimate the budgetary consequence of adding Ustekinumab to the reimbursement drug list in Thailand. METHODS: From payer’s perspective, a 5-year budget impact model was developed based on the current treatment algorithm and reimbursement criteria for moderate to severe chronic plaque psoriasis. We assumed the current biologic reimbursement program for psoriasis in civil servant medical benefit scheme (CSMBS) was expanded to cover all Thai population. Annual budget impact assessment was a result of changes of current mix of biologic therapies by introduction of Ustekinumab. This use is as 1st or 2nd biologic agent in patients with inadequate response from systemic therapies or other biologics, respectively. Current and future market mix data of biologic therapies was estimated by experts’ opinions, and products’ efficacy and epidemiology data was derived from literature. Cost variables included medical care costs: biologic acquisition costs, physician visits, monitoring lab tests and inadequate response care. Univariate sensitivity analysis was conducted to assess the model robustness. RESULTS: Ustekinumab’s inclusion showed the average total budget spending of 260 million baht per year (226 – 309 million baht from 1st - 5th year respectively). Comparing to current scenarios with two biologics available, we found the diminution of total budget around 81 million baht in 5-yr period, accounting for 0.14 million baht per patient. Average budget growth rate was slightly lower with the inclusion of Ustekinumab (9.90% vs. 11.75%).  Comparative total cost per responder was found lowest in Ustekinumab followed by Infliximab and Etanercept respectively. These results were mainly driven by the number of patients on biologics and distribution of patients among the available therapies. CONCLUSIONS: The addition of Ustekinumab to the current biologic reimbursement list provides more efficient management approach for the treatment budget of moderate to severe chronic plaque psoriasis in Thailand.