Phenylketonuria (PKU) is an inherited metabolic disorder that occurs due to impaired oxidation of phenylalanine (Phe) to tyrosine. It is inherited in an autosomal recessive pattern, with incidence of around 1:10 000 in European population. It is caused by mutated gene for the liver enzyme phenylalanine hydroxylase (PAH). PAH deficiency leads to hyperphenylalaninemia which has negative impact on brain. Clinically, patients with PKU can present with wide range of symptoms, depending on the quallity of disorder management. These patients can have delay in psychomotor development, slowed mental and somatic development, changes in behaviour, epileptic seizures, hyperactivity, motoric disbalans and many other symptoms. Phenotypic features of these patients include skin, hair and eye hypopigmentation as well as characteristic mousy odor of sweat and urine due to phenyllactic acid. In pregnant women with poorly regulated PKU, hyperphenylalaninemia has teratogenic effects on the fetus and there is an elevated risk for mental retardation, microcephaly, congenital heart disease and intrauterine growth restriction. Newborn screening (which is mandatory in Republic of Croatia) is a standard way to discover PKU. The type of disorder is usually determined by the Phe plasma concentration. Most common type in Croatia is classical PKU (plasma Phe>1200 μmol/L), however less severe forms also exist. Therapy consists of strict low-protein diet which is implemented right after the confirmed diagnosis and must be conducted throughout the whole life. Therapeutic goals are normal intelectual and somatic development of the child. This paper presents a case of an 7 year old boy with PKU which has been poorly regulated since birth (despite timely set diagnosis). Due to social status of family and local community, care for the child was not adequate: PKU diet hasn't been correctly implemented, there was no continuity in doctor visits and there was lack of interest for education about PKU. Unfortunate circumstances during patients early childhood have resulted in drop of IQ and delay in neurocognitive development.