The normal value for phenylalanine concentration in children is <120 μmol/L (2 mg/dl). Phenylpropionic acid concentration in children is measured clinically, and less than 120 μmol/L is normal; between (120 and 360) μmol/L is mild phenylketonuria; between (360 and 1200) μmol/L is moderate phenylketonuria; and greater than 1200 μmol/L is severe phenylketonuria. In children with phenylketonuria, mental retardation is the most prominent feature. The condition is usually normal at birth, and if not treated with dietary therapy, skin and hair changes and restricted growth and development may occur. Active treatment should be given as early as possible, and the younger the age at which treatment is started, the better the effect. Treatment is mainly through dietary therapy, such as a low-phenylalanine diet. Children with abnormalities in the tests for phenylketonuria are advised to seek prompt medical attention to avoid delays.