Sapropterin dihydrochloride (Kuvan)) is a synthetic formulation of the active 6R-isomer of tetrahydrobiopterin, a naturally occurring co-factor for phenylalanine hydroxylase. In the EU, sapropterin is approved for the treatment of hyperphenylalaninemia in patients > or =4 years of age with tetrahydrobiopterin-responsive phenylketonuria (PKU), and in adults and children with tetrahydrobiopterin deficiency who have been shown to be responsive to such treatment. In the US, it is approved to reduce blood phenylalanine levels in patients with hyperphenylalaninemia due to tetrahydrobiopterin-responsive PKU. Oral sapropterin effectively lowers blood phenylalanine levels in a proportion of patients with PKU; to date, there are no published efficacy trials of the specific sapropterin formulation under review in patients with tetrahydrobiopterin deficiency. Sapropterin was well tolerated in patients with PKU, although longer-term tolerability data are required. Sapropterin is the first non-dietary treatment for patients with PKU that has been shown in randomized, double-blind trials to be effective in lowering blood phenylalanine levels. Thus, sapropterin provides a promising treatment option for patients with PKU who are tetrahydrobiopterin responsive.