Resumen La fenilcetonuria pertenece al grupo de las hiperfenilalaninemias. Es una enfermedad infantil, metabolica, causada por un deficit de la enzima (sustancia proteica capaz de activar indirectamente una reaccion quimica definida) del higado llamada fenilalanin-hidroxilasa (1). La alteracion en el metabolismo del aminoacido llamado fenilalanina, sobre el que actua la enzima deficiente, provoca un acumulo excesivo de fenilalanina en los liquidos del organismo (1). La fenilcetonuria es una enfermedad progresiva severa que puede producir retraso mental si no se trata con una dieta libre de alimentos que contengan este aminoacido a tiempo (3-5). Palabras clave: Fenilcetonuria, hiperfenilalaninemia, fenilalanina, fenilalanin-hidroxilasa. Abstract Phenylketonuria (PKU) is one of a range of hyperphenylalaninemias, of differing etiology and severity, revealed by the mass screening of newborn blood phenylalanine concentration (1). Classical PKU is characterized by a markedly elevated blood phenylalanine concentration (1), as a consequence of the total lack of the hepatic enzyme phenylalanine hydroxylase. Increased circulating concentration of phenylalanine, if untreated, results in a range of physical, physiological and psychological aberrations, the major one being mental retardation (1). Once diagnosed, treatment by dietary phenylalanine restriction should be initiated promptly, since delays have been correlated to increased severity of retardation (3-5). Key words: Phenylketonuria, hyperphenyalaninemia, phenylalanine, phenylalanine hydroxylase.