Tetrahydrobiopterin (BH4) deficiency accounts for 1-3% of all reported forms of hyperphenylalaninaemia (HPA). Deficiencies of two enzymes, GTP-cyclohydrolase (GTPch; EC 3.5.4.16) and 6-pyruvoyl tetrahydropterin synthase (6-PTS), lead to a defect in BH4 biosynthesis. During the hydroxylation of phenylalanine (Phe) to tyrosine by phenylalanine hydroxylase (PAH; EC 1.14.16.1), BH, is oxidized to quinonoid dihydrobiopterin (qBH2). In order to regenerate BH4, qBH2 is reduced by the enzyme dihydropteridin reductase (DHPR; EC 1.6.99.7). More recently a new form (primapterinuria) due to a possible defect in the activity of dehydratase has been described. Since BH4 is required as cofactor by PAH and by mammalian aromatic amino acid hydroxylases, a deficiency leads not only to HPA but also to a deficiency of biogenic amine neurotransmitters dopamine and serotonin (Blau 1988), its clinical picture and mode of therapy would be expected to be different from the other types of HPA.,This paper describes distinctive clinical and biochemical features of 16 BH4-deficient cases diagnosed and followed up at a single metabolic unit.