Background: Phenylketonuria is a kind of disorder in phenylalanine metabolism which is caused by hepatic enzyme deficiency that transmitted in autosomal recessive mode. The prevalence is 1 in every 12000 birth and mental retardation is its prevalent phenomenon. Delay in language and cognition development is significant among the effects of mental retardation that will lead to communication disorder. Since the most important means of communication is language, it is likely that language impairment in patients with Phenylketonuria is caused communication disorder.

Method: Two children with Phenylketonuria were studied, a girl (7.3 ys) and a boy (8.11 ys). The standard score in three areas of picture vocabulary, oral vocabulary and conjunction vocabulary was used to determine the semantic use. Finally, standard scores and the semantic use of cases compared with age-matched healthy children.

Results: The first case averagely in 3.1 ys shows semantic delay with the decrease in semantic point from 106 to 81.The second cases averagely in 5.5 ys shows semantic delay with the decrease in semantic point from 107 to 71.

Conclusion: Observations demonstrate noticeable semantic delay among both cases.