Vitamin level in blood plasma and erythrocytes and a rate of urinary excretion of vitamin metabolites were analised in children suffering from phenylketonuria. It has been shown that vitamin B1 metabolism and therefore the criteria of the adequate sufficiency with this vitamin does not differ from those for healthy people. Increased riboflavin urinary excretion under its decreased plasma and erythrocyte levels has been demonstrated for PKU children. In consequence of this the indexes of sufficiency significantly differ from those of the healthy adequatly supplied with this vitamin children and are equal to 4 ng of riboflavin per 1 ml of blood plasma and its urinary excretion more than 50 mg/h. The necessity for the redetermination of vitamin B2 diet optimal content under this disease and its biochemical validation are discussed.