Correlation between the state of vitamin metabolism and the impairments in carbohydrate, lipid and protein metabolism was studied in 35 children of 9-13 years of age with diabetes mellitus of various severity standing for up to 7 years. Deterioration of riboflavin metabolism in insulin-dependent diabetes mellitus, expressed as an increase of the vitamin excretion with urine, was augmented with prolongation of the disease duration; the deterioration was sometimes related to the value of glycemia and glucosuria, being the indicative symptom of the disease. In spite of some limitations in validity of experiments related to insufficient number of children in some groups, a decrease in excretion of 1-methyl nicotinamide with urine was detected in all the children with the comatose state, in acidoketosis and glucosuria (above 20 g/day), whereas normal content of nicotinamide coenzymes was found in erythrocytes. Deficiency in vitamins B1, B6 and C was observed more often (5-100%) in children with elevated content of cholesterol as compared with 7-67% of children exhibiting normal level of cholesterol. Optimization of vitamins B and C consumption in children as well as use of any means for correction of these vitamins deficiency are discussed.