Background: Phenylketonuria (PKU) is an autosomal recessive (AR) disorder of phenylalanine (Phe) metabolism. Increased blood Phe levels could be associated with harmful actions on the cerebral function. Newborn screening (NBS) programs allow proper recognition and management of PKU with low-Phe diet. Objective: This study aimed to assess neurological development, behavioral and dietary regimen of patients with phenylketonuria with positive neonatal screening compared to patients with phenylketonuria without neonatal screening. Patients and methods: This observational study was conducted on a total of 40 children with PKU and were divided into two groups: Screening group (n=27) and non-screening group (n=13). All patients were subjected to complete history taking, special dietary regimen, developmental milestone examination, neurological examination, IQ test assessment, behavior change evaluation and laboratory investigations. Results: There were significant increases in motor affection hyperreflexia and developmental millstones in non-screening group. ADHD demonstrated insignificant difference between both groups. There was significant increase in phenylalanine level in non-screening group compared to screening one before treatment and 6 months after treatment, while no significant difference was recorded at 12 months after treatment. There was a statistically significant correlation between Phe level and IQ before and 12 months after treatment. IQ was higher among non-ADHD cases compared to attention deficit hyperactivity disorder (ADHD) ones. Conclusion: The current study concluded that PKU still has adverse effects on children in the context of motor function, developmental milestone, ADHD development and IQ affection. Early screening seemed to be associated with promising outcomes.