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Phenylketonuria (PKU) is one of the aminoacid metabolism disorder, which affect functioning of the phenylalanine hydroxylase enzyme and cause increasing level of phenylalanine. This excessive level may cause reduction of myeline, neuronal loss and decreased levels of interneuronal connections and neurotransmitter density, damaging the brain. Consequently, PKU is frequently associated with intellectual disability, seizures, delayed development, behavioral problems, and psychiatric disorders autistic symptoms. We report a case of PKU presenting with neurological impairments including autistic behaviors: A 9-month-old girl was admitted our outpatient clinic by her mother with complaints of inability to sit supported. At the first clinical evaluation she had poor eye contact, she did not smile reciprocally. She could not follow object by eye gaze, could not return to the side. She had seizures. The patient was consulted to neurology and she was diagnosed as PKU. Neonatal screening was performed but her blood sample confused with other girls' whose name was same. 2 Months later, at the second clinical assessment after she was started phenylalanine restricted diet, she began to follow object by eyes, had longer eye contact and could smile reciprocally. She had no seizure. The number of phenylketonuria cases are declining recent years because of newborn screening but in some the diagnosis may miss due to several reasons including births outsides hospitals, too early screening. Physicians should be aware of PKU when confronted with case with neurologic impairments and autistic behavior, so if the the treatment starts early enough, a normal neuropsychological development would be caught, preventing presentation of brain dysfunction in the child, which sometimes include the autistic features.