The plasma amino acid concentrations were measured in 10 children with phenylketonuria (PKU) on the phenylalanine-free amino acid mixture Phenyldon and 9 PKU children on the phenylalanine-free protein hydrolysate Albumaid. The blood samples were taken fasting and two hours after the meal. The individual amino acid concentrations, the glycine/valine ratios, and the total amino acid concentrations were mutually compared and also compared to reference intervals. No differences of the concentration of phenylalanine on the two diet formulas were observed. Children on Phenyldon showed fasting and postprandial isoleucine higher than the reference intervals. Furthermore, a postprandial increase of several amino acids was observed on both diet formulas. This response was statistically significant in children on Phenyldon.