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Background and Aim: Phenylketonuria (PKU) is an inherited metabolic disease. The aim of this study was investigation the intelligence and language function in children with PKU based on the age of treatment onset and quality of dietary control.

Methods: In this cross-sectional study, the language and intelligence of 31 children with PKU at the age of 4 to 6.5 years were investigated by test of language development-third edition ( TOLD-3) and Wechsler preschool and primary scale of intelligence (WIPPSI), respectively the data compared with of 42 normal children. Patients were classified into 2 groups with different level of dietary control as good and poor [serum phenylalanine (Phe) level less and more than 6 mg/dl, respectively].

Results: There were significantly differences (p=0.001) between children with PKU and controls in intelligence and language scores. Early treatment improved significantly the intelligence and language scores in comparison with late treatment (p=0.019). Comparing late treatment before and after the age of 1 year showed that late treatment until 1 years of age improved significantly (p=0.050) all of functions except performance intelligence quotient (IQ). In early treated PKU, good dietary control improved significantly (p=0.021) and also, verbal and total IQ in comparison with poor dietary control.

Conclusion: Although, the language development in children with PKU is affected by age of treatment onset, cognitive development features such as verbal and total IQ is affected not only by the time of treatment onset but also by the quality of dietary control.