Phenylketonuria (PKU) is an inborn poly-enzymatic multi-system pathology of the metabolism, its primary block being the Phenylalanine hydroxylation that leads to severe mental retardation with clinical and biochemical polymorphism. 30 PKU children and 31 Phenylalanineuria children have been investigated for the level of free amino acids in blood and urine. The troubles of the metabolism of Phe, Trp, Met, urea cycle and of enzyme transport systems on the kidney level were appreciated in both studied lots. The effectiveness of the PKU treatment increases if combining the low Phe diet with drug metabolic correction on the base of free amino acids level in blood and urine.