Investigations were made on the relationship between riboflavin concentration of the blood in various diseases and on the variation of it after some therapeutic treatments. The average concentration of esterified riboflavin in the blood was lower in a variety of diseases than that of healthy subjects and the difference was at a significant level. Especially in hepatic cirrhosis and carcinoma of the liver or bile ducts, the lowest values were obtained. It was also lower in the blood of those who showed a positive reaction in hepatic examination, with the exception for hepatic tumor, than in that of negative reaction. Total riboflavin in the blood was lower in the subjects whose leucocyte was below 6,000 than in those over 6,000. A marked increase of esterified riboflavin in the blood was always recognized after the administration of ATP with or without thiamine or riboflavin. This fact might be very significant in the treatment for severe riboflavin deficiency.