The aim of the study is to confirm the efficacy and safety of BH4 in the treatment of hyperphenylalaninemia caused by phenylalanine hydroxylase deficiency in patients responsive to BH4. The primary objective is to assess the effect of BH4 on phenylalanine tolerance compared to placebo under optimal blood phenylalanine control and to demonstrate safety in 12 months long-term treatment. Additionally population PK will be assessed.