Inborn errors of metabolism affect the metabolism of 7 out of 8 essential amino acids and a number of non-essential ones. Dietary treatment has been applied with varying success. The wide variations in the severity of symptoms in this group of diseases are discussed. Dietary treatment opens the possibility to collect information about the minimal requirements of most essential amino acids. These requirements are likely to be still lower than those mentioned in the WHO Report No. 522, 1973. Children who for years have lived on one of these restricted diets, provide the possibility to compare the effect of such a diet on their nutritional status with that of children living on a normal diet.