Whether specific cognitive deficits related to frontal-lobe dysfunction that have been reported in individuals with phenylketonuria (PKU) are also characteristic of mild hyperphenylalaninemia (MHP) was investigated. Tests of executive function and control tasks not assessing executive function were administered to a group of individuals with MHP and a group without MHP, similar in age, gender, and IQ. Tests of academic skills and behavior-rating questionnaires were also administered to the group with MHP. No group differences were found for any measure, suggesting that the mild elevations of phenylalanine in individuals with MHP are not sufficient to produce behavioral and cognitive impairments characteristic of PKU.