Levels of serum 1, 5-anhydroglucitol (1, 5-AG) are known to fall in proportion to urinary excretion of glucose in patients with NIDDM. It has been also reported that urinary excretion of 1, 5-AG increases in hyperglycemic patients. However, renal tubular reabsorption of 1, 5-AG has not yet been investigated. In order to demonstrate the relation between glucosuria and renal tubular reabsorption of 1, 5-AG, we determined urinary glucose, serum 1, 5-AG levels and fractional excretion of 1, 5-AG (FE-AG) up to 47 times in 24 patients with NIDDM. FE-AG correlated with urinary glucose (r=0.651, P<0.001), which indicates that renal tubular reabsorption of 1, 5-AG decreases in hyperglycemic patients. There was an inverse correlation between serum 1, 5-AG and FE-AG (r=-.681, P<0.001), which suggests that an increase in FE-AG contributes to a fall in serum 1, 5-AG. FE-AG well demonstrated renal handling of 1, 5-AG and also accounted for changes in serum levels of 1, 5-AG in diabetic patients.