The aim of treatment of phenylketonuria is to restore and then to maintain amino acid balance in spite of the metabolic defect. This paper presents the details of management of the low-phenylalanine dietary regimen with particular emphasis on its safe application in infancy. Individualized prescription and monitoring systems represent fundamental improvements in the dietary treatment program. Management of treatment requires a high degree of understanding of the related biochemical responses and also requires the combined skills of an interdisciplinary metabolic team and the parents. It is particularly important at this time that precision in treatment be emphasized as strongly as detection of the disease. This scheme of nutritional, biochemical, and psychological management has proved to work well and to be practical.