Phenylketonuria (PKU) is caused by a deficiency or inactivity of the enzyme phenylalanine hydroxylase that converts phenylalanine (Phe) to tyrosine (Tyr). It has been proposed that a reduction of brain Tyr levels, as well as reduced activity of the key regulatory enzyme of dopamine (DA) synthesis tyrosine hydroxylase, leads to a depletion in DA activity in patients with PKU. We report a case of a 56-year-old woman with an intellectual disability due to late diagnosis of PKU and parkinsonism, with a modest clinical response to levodopa therapy.