The methods for the determination of protein requirements are reviewed and the difficulties in achieving the recommendations of the dietary management of phenylketonuria proposed by a Medical Research Council Working Party on Phenylketonuria using currently available low phenylalanine (Phe) protein substitutes and low protein foods are examined. These recommendations are that all infants whose blood Phe concentrations exceed 600 µmol/1 in the presence of a normal or low plasma tyrosine and an otherwise normal plasma amino acid profile while receiving a normal protein intake (2–3 g/kg/day), should start a low Phe diet immediately. Infants whose blood Phe concentrations remain persistently between 400 and 600 µmol/1 for more than a few days should also start treatment. The diet should contain a protein substitute which is Phe free (or at least very low in Phe) and otherwise nutritionally complete with a composition sufficient to provide 100–120 mg/kg per day of tyrosine and a total amino acid intake of at least 3 g/kg per day in children under 2 years of age. In children over 2 years the intake of amino acids should be maintained at a level of 2 g/kg per day. The protein substitute should be spread as evenly as possible through the 24 h. Blood Phe concentrations should be maintained between 120 and 360 µmol/1. In children aged over 10 years it is suggested that the protein substitute should supply the protein reference nutrient intake + 50%. An upper blood Phe limit of 480 µmol/1 rather than 360 µmol/1 may be acceptable in school age children. Adults and adolescents should continue treatment with the aim to maintain blood Phe concentrations no higher than 700 µmol/1. During the period before conception and during pregnancy women should aim to have plasma Phe concentrations between 60–250 µmol/1.