Background: The aim of the study was to determine neurological and neuropsychiatric manifestations in a group of patients with previously undiagnosed or untreated phenylketonuria (PKU) in the south-eastern part of Poland. Methods: The study was conducted among 400 adults with severe intellectual disability who were born prior to neonatal screening and who resided in Social Welfare Homes in south-eastern Poland. Participants were aged between 27 and 89 years (mean 51 years). The screening for PKU was performed by blood test using tandem mass spectrometry method (MS/MS) and analysis of organic acids in urine. Among all new identified untreated individuals, neurological symptoms were recorded and neuropsychiatric disturbances were assessed with The Neuropsychiatric Inventory-Nursing Home Version (NPI-NH). Results: We found 10 previously undiagnosed and one previously diagnosed but untreated individuals with PKU (four women and seven men). The most commonly exhibited neurological manifestations included the following: profound speech disturbances, balance disturbances, muscular hypertonicity, muscular tremors and seizures. The most frequent behaviours included the following: aberrant/troublesome motor behaviour, agitation/aggression and euphoria/elation. Conclusion: Data from the present study indicate that future treatment of these individuals should combine a dietary intervention with individual behavioural therapy and neurorehabilitation to improve general health and quality of life.