The LAT1 amino acid exchanger is responsible for phenylalanine transport across the blood-brain barrier. As phenylalanine excess in the brain leads to mental retardation in untreated patients with phenylketonuria, mutations of the LAT1 gene can be responsible for milder clinical manifestations (lower brain toxicity of hyperphenylalaninaemia) observed in some patients. The aim of this study was to screen for LAT1 gene mutations in phenylketonuric patients. Within the coding sequence of the gene, three polymorphism and one new mutation (G41D) were detected. The protein sequence change due to the mutation described could influence the clinical course of phenylketonuria.