The aim of this study was to compare the level of maternal knowledge and the blood phenylalanine (Phe) control in phenylketonuria (PKU; OMIM 261600). The study was conducted on 144 children (81 boys, 63 girls) with PKU, aged between 1 and 15 years, at Hacettepe University Ihsan Dogramaci Children's Hospital, Metabolism and Nutrition Unit. All subjects were treated with a low-Phe diet using a Phe-exchange system. A 20-question multiple-choice questionnaire was applied to the mothers to determine their knowledge about PKU and its dietary treatment. Questions in the test consisted of the knowledge about the disease (6 questions), general dietary knowledge (14 questions) and knowledge about specific exchange within the dietary questions (6 questions). The median blood Phe concentration for the previous 3-year period was used as an indicator of metabolic control. Eighty-seven children had a median blood Phe concentration above the MRC Working Party Guidelines. There was a negative correlation between maternal knowledge about exchange and median blood Phe concentration in the child (p<0.05). Maternal knowledge about a standard 15 mg Phe exchange system is correlated with dietary compliance as measured by blood Phe concentrations in our subjects. We would like to implement an easier method of measuring Phe exchanges to improve dietary knowledge in the mothers.