The mainstay of PKU treatment is a low-phenylalanine diet (i.e restriction of natural protein), and supplementation with a protein substitute (a mixture of amino acids free from phenylalanine, also containing micronutrients and vitamins) and special low-protein foods, to meet the patient's energy requirements. When diet and treatment is relaxed after childhood, adult and adolescent patients with phenylketonuria are at risk for malnutrition, depending on the compliance with treatment and the intake of amino acid supplements.

In this study, nutrition status of patients with PKU and hyperphenylalaninemia is systematically assessed under ongoing current treatment, in relation to Phe-tolerance, compliance with treatment, and psychosocial issues.

Study participants do not undergo any specific therapeutic or diagnostic intervention.

Inclusion criteria:

• PKU (Phenylketonuria) or hyperphenylalaninemia
• male or female, minimum age 16y
• signed informed consent

Exclusion criteria:

- patient who can not comply with the study protocol (e.g. unable to fill in questionnaires by themselves or a representative)

Patients with phenylketonuria or hyperphenylalaninemia, in treatment at the participating centres