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Phenylketonuria (PKU), an autosomal recessive disease, is the most common inborn error of amino acid metabolism in Caucasians, affecting 1/10,000 individuals. PKU is caused by the deficiency of hepatic phenylalanine hydroxylase (PAH), which catalyzes the hydroxylation of phenylalanine (Phe) to tyrosine, using tetrahydrobiopterin (BH4) as a cofactor. The PAH gene is located on the chromosome 12 and consists of 13 exons. Over 600 different mutations of the PAH gene have been identified to date, which result in a broad spectrum of PAH deficiency. The resulting elevation of Phe in the blood (hyperphenilalaninemia – HPA) could cause mental retardation if left untreated. The classification of PKU is based on the metabolic phenotype of a patient (according to HPA level); discerned could be three subclasses of PKU (classic, moderate, mild) and mild HPA, which is a separate clinical entity. The incidence of classical PKU in the Slovene population was estimated to be 1/10,000, corresponding to a carrier frequency of about 1/50. The cumulative incidence of all subtypes of PKU (classic, moderate, mild) is around 1/6,000; the incidence of mild HPA is around 1/3,500. The article also reviews the previously published studies on the genetic and phenotypic characteristics of Slovenian PKU patients, performed at the Department of Pediatric Endocrinology, Diabetes and Metabolism, University Children’s Hospital Ljubljana, in years 2008–2012. The genetic characteristics of the Slovenian PKU population were concordant with other neighbouring populations; five novel mutations of PAH gene were detected in the population. The mandatory neonatal PKU screening in Slovenia was implemented in 1979. The dietary therapy based on a restricted Phe intake should be introduced as soon as possible after birth; in responders, BH4 treatment increases the dietary Phe tolerance.