Phenylketonuria (PKU) is a metabolic disorder that is characterized by decreased activity of phenylalanine hydroxilase and has an autosomal recessive inheritance. Mental retardation, learning difficulties, behavioral problems, hyperactivity and attention deficit are frequently seen mental problems in PKU. The co-existence of PKU and autism spectrum disorders (ASD) is documented in the literature. Newborn screening allows early detection of PKU and special diet offers enormous benefits to these children. Here we present a case with ASD, whose PKU diagnosis was omitted during newborn screening. A 8-year-old male referred from child neurology to our department with complaints of limited eye contact, stereotypic speech and movements, and odd interests. The patient was admitted to child neurology department with febrile convulsion when he was 2,5 years old. When he got three years and a half old, the diagnoses of epilepsy, autism and severe mental retardation were made and he started to get special education. During the search for a better treatment by the family, the diagnosis of PKU was set in a private hospital. Diet treatment was commenced; however they discontinued this treatment because of severe intolerance. Symptoms of echolalia, stereotypic hand movements were seen during psychiatric assessment. Irritability, hypersensitivity to sound and unstoppable crying were documented. His difficulty of comprehension was evident and self-care skills were absent. The patient was diagnosed to have autistic disorder and severe mental retardation according to DSM-IV-TR. He attended special education program and took several antipsychotic medications. However, symptoms did not improve despite relatively high doses. The association between PKU and ASD were reported in several studies in the literature. It was discussed that increased ASD risk in PKU may be resulted from late diagnosis of PKU, however, this association was weakened after developments in treatment in PKU. Therefore, in children with epilepsy, mental retardation and ASD, whose symptoms are resistant to treatment, the diagnosis of PKU should be considered.