OBJECTIVES:  Routine testing for HLA-B*5801 in European patients has been proposed before allopurinol treatment aiming to reduce the incidence of Stevens-Johnson syndrome/toxic epidermal necrolysis (SJS/TEN). Having some discretion in the allocation of financial resources, Portuguese NHS hospitals may be particularly interested in this issue as extra costs with the treatment of SJS/TEN episodes may be avoided. This study aims to evaluate the cost-effectiveness of HLA-B*5801 genotyping versus no genotyping (usual care) in Portuguese adult patients newly diagnosed with gout before allopurinol treatment. METHODS: A decision tree model was constructed to structure the sequences of clinical events that may occur after the diagnosis of the disease, over a period of three months. Effectiveness data were estimated for Portuguese patients based on published literature and Portuguese population databases. All costs were obtained from official prices published for the Portuguese NHS. Admitting that NHS hospitals objective is to lower costs in the treatment of gout, the incremental cost per SJS/TEN reaction avoided was the  incremental cost-effectiveness ratio (ICER) considered. One way sensitivity analysis was performed for each relevant parameter. RESULTS: The use of HLA-B*5801 genotyping versus no genotyping was associated with a decrease of 1.24 SJS/TEN reactions and an incremental total cost of 42,401.03€, resulting in an ICER of 34,194.38€ per SJS/TEN reaction avoided.  Sensitivity analysis results showed that base-case results are very sensitive to variations in the baseline odds ratio of gene-disease association, epidemiological data used to estimate the incidence of SJS/TEN and genotyping cost. CONCLUSIONS: Widespread use of HLA-B*5801 genotyping in Portuguese hospitals is not cost-effective at the current unit price of 45.30€ as it leads to a substantial increase in expenditure compared to usual care. However, it may be an advantageous option if its maximum price would equal 6,37€.