Introduccion: la fenilcetonuria (PKU) es un error del metabolismo de los aminoacidosdebido a la mutacion del gen que codifica para la enzima fenilalanina hidroxilasa (PAH). Se han reportado 564 mutaciones en diferentes exones. Tiene patron de herencia autosomica recesiva y genera graves repercusiones neurologicas y sistemicas, si no se detecta en etapa neonatal. Caso clinico: paciente masculino de 19 anos con diagnostico PKU clasica a los 8 anos de edad. Producto de embarazo a termino, padres no consanguineos, curso con retraso psicomotor, retraso en lenguaje, deterioro cognitivo y pobre adaptacion social. Se determinaron dos tipos de mutaciones: una grave S349P y una leve L348V. Niveles de fenilalanina (Fen) en sangre elevados (26.12 mg/dL o 1567.2 μmol/L), que permiten clasificar como PKU clasica. Conclusion: es importante realizar de forma temprana estudios moleculares que permitan correlacionar el genotipo con el fenotipo, para predecir la gravedad de la enfermedad y orientar el tratamiento. Rev.cienc.biomed.2013;4(2):339-344 PALABRAS CLAVES Fenilcetonuria; Fenilalanina; Mutacion; Fenilalanina hidroxilasa; Trastornos neurologicos. SUMMARY Introduction: Phenylketonuria (PKU) is an error of the metabolism of the amino acids due to the gene mutation that codifies for the enzyme called phenylalanine hydroxylase (PAH). 564 cases of mutations in different exons have been reported. It has an autosomal recessive inheritance pattern and generates severe neurological and systemic consequences if it is not detected in the neonatal stage. Case report : 19-year-old man who was diagnosed with PKU at the age of 8 years, product of at term pregnancy, with non-consanguineous parents. He presented psycho- motor retardation, language delay, cognitive deterioration and poor social adaptation. Two types of mutation were determined: A severe type S349P and a mild type L348V. High serum phenylalanine levels (26.12 mg/dL o 1567.2 μmol/L) allow classifying as classic PKU. Conclusion: It is important to carry out early molecular studies that allow correlating the genotype with the phenotype, which permit to predict the severity of the disease and to orientate to the correct treatment. Rev.cienc.biomed.2013;4(2): 339-344 KEYWORDS: Phenylketonuria; Phenylalanine; Mutation; Phenylalanine hydroxylase; Neurological disorders.