Introduction: Tetrahidrobiopterin (BH4) is an essential cofactor for phenylalanine, tyrosine and tryptophan hydroxylase and its deficiency results in hyperphenylalaninaemia and depression of neurotransmitters in the central nervous system. It is caused by defects in one of four enzymes responsible for its synthesis or regeneration. Typically manifests in the neonatal period or after the first year of life with motor delayed and / or cognitive impairment, hypotonia, movement disorders, seizures or autonomic symptoms. Objective: To describe a case of tetrahidrobiopterin deficiency following in the Clinic of Metabolic Diseases, Hospital das Clinicas USP. Case Description: Male, 3 years old, son of non-consanguineous parents, with no relevant gestational history, when he was asymptomatic with 7 days of life received neonatal screening for phenylketonuria changed. Initiated dietary restriction without a satisfactory answer. Evolved with hypotonia, developmental delay, gait ataxia and choreiform movements in hands. Introduced sapropterin with symptom improvement, currently presenting language delay. Biopterin and neopterin undetectable in urine and normal dihidropterin reductase activity. Tomography and magnetic resonance of brain unchanged. Discussion / conclusion: The tetrahydrobiopterin metabolism disorders are rare forms of hyperphenylalaninaemia, representing 2% and make differential diagnosis with phenylketonuria, but more severe, worse therapeutic response and substantially different treatment. The suspected diagnosis by neonatal screening with subsequent early confirmation by specific laboratory tests are extremely important because they allow proper treatment and prevention of progressive neurological damage.