Phenylketonuria is an inherited metabolic disorder that leads to neurobehavioral dysfunction. The main treatment is a low-phenylalanine diet and/or the cofactor tetrahydrobiopterin. Regular outpatient follow-up care and measurement of the phenylalanine level in the blood are required. We aimed to analyze the economic burden of phenylketonuria on families and the state. The pa-tients with phenylketonuria were divided into three groups according to their treatment: a low-phenylalanine diet group (n= 50), a tetrahydrobiopterin group (n= 44) and a group taking tetra-hydrobiopterin together with the diet (n= 25). A comparative cost analysis was carried out. The annual economic burden to the state was calculated to average €18801±15345 and was lowest in the diet group, then in the tetrahydrobiopterin group and highest in the tetrahydrobiopterin+diet group (p < 0.001). Out-of-pocket costs amounted to €1531±1173 per year, and indirect losses aver-aged €2125±1930 per year for all families. The economic loss was significantly lower in the fami-lies taking tetrahydrobiopterin than in the other groups (p=0.001). The combined use of medical nutrition and BH4 treatments has been shown to increase the economic harm to the state. Reim-bursing low-protein products and increasing the rate of patients eligible for financial allowances may reduce the economic harm to families.