The same basic principles are used to deliver dietary treatment in PKU that was developed sixty years ago. Dietary treatment is undoubtedly very successful, but it has gradually evolved and been guided commonly by individual experience and expert opinion only. There is little international consensus about dietary practice with improvements in specialist dietary products concentrating on taste and presentation rather than nutritional composition. Many areas of dietary treatment have not been rigorously examined. In particular, the amino acid and micronutrient profile of Phenylalanine-free (phe-free) amino acids requires further study. In different formulations of phe-free amino acids, there are variations in the amino acid patterns as well the amount of essential and non essential amino acids per 100g/amino acids. The amount of added tyrosine and branch chain amino varies substantially, and in PKU specifically, there is little data about their relative absorption rates and bioavailability. In phe-free amino acids, there is evidence suggesting that some of the added micronutrients may be excessive and so the source and amount of each micronutrient should be scrutinized, with a need for the development of international nutritional composition standards exclusively for these products. There is a dearth of data about the life-long phenylalanine tolerance of patients or the nutritional state of adult patients treated with diet. There is a growing need to measure body composition routinely in children with PKU and with the rise in childhood obesity, it is important to measure body fatness and identify those who are at greatest risk of 'co-morbidities' of obesity. There is necessity for international collaboration to ensure robust data is collected on many basic aspects of nutritional care to guarantee that diet therapy is delivered to the highest standard.