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Objective: Phenylketonuria (PKU) is one of the most common metabolic disorders worldwide. If left untreated, it causes neuropsychiatric sequelae, with seizures being a common occurrence. There is little information about the clinical features of epilepsy, electroencephalography (EEG) findings, and factors related to seizure outcomes in adult patients. We aimed to investigate these variables in adult PKU patients. Methods: We conducted a retrospective search in our database using the keywords “PKU and epilepsy” for the period between 2008 and 2022. Demographic, clinical, EEG, and cranial magnetic resonance imaging (MRI) findings of the patients were extracted from the electronic health records. Scalp EEG and MRI findings were reassessed. Phenylalanine (Phe) levels of the cases were retrieved. The potential correlation between seizure outcome and laboratory findings was analyzed. Results: Ten patients (4 females; aged: 19–55 years) were included. Seizure onset was various. The most common seizure type was bilateral tonic-clonic. Nine patients were on antiseizure medications (ASMs); seven were seizure-free. EEG background activity was slow in four patients, with paroxysmal discharges in eight individuals. The most frequent MRI finding was periventricular white matter hyperintensity. No correlation existed between seizure outcome and clinical, EEG, MRI results, or Phe levels. Seizure freedom was more common in patients with good dietary compliance. Conclusion: Bilateral tonic–clonic seizures were the most common type of seizure, accompanied by frequent paroxysmal activity in EEG. MRI scans revealed periventricular white matter hyperintensity. Seizure freedom was commonly achieved with ASMs, irrespective of blood Phe levels. Nevertheless, dietary compliance may play a role in seizure control.