Abstract: The invention relates to an improved tetrahydrobiopterin formulation for treating patients affected by hyperphenylalaninemia. More specifically, the formulation of the invention permits a prolonged residence of tetrahydrobiopterin in the plasma, and/or a higher concentration of sapropterin in the plasma, enabling notably the treatment of HPA patients that would usually not be classified as responders to tetrahydrobiopterin loading test, and therefore left without any pharmacological treatment opportunity, but only with a strictly controlled protein diet.