Phenylketonuria (PKU) is a disorder caused by an inborn error of metabolism, causing cognitive and behavioral disorders when not treated. Heterozygotes (i.e., patients' parents) were described with low verbal intelligence quotient, but no study systematically investigated cognitive functions in PKU parents. To obtain a neuropsychological profile in heterozygotes, we compared cognitive performance of heterozygotes and healthy controls (HC) on cognitive battery. Twelve heterozygotes and 14 HCs underwent standardized neuropsychological tasks assessing frontal/executive functions, memory, and visuospatial abilities. No significant difference between heterozygotes and HC was found on demographic aspects. Heterozygotes performed worse than HC on immediate verbal recall, on test assessing set-shifting, divided attention, and sensitivity to processing speed. No difference was found on the remaining cognitive tests.In conclusions, we observed less efficient control/executive functions in heterozygotes when compared to HCs. Further studies in large sample of heterozygotes should be performed to confirm our results.