The urinary excretion of 1,5-anhydro-D-glucitol, a pyranoid polyol, in humans was studied. The plasma of nondiabetic human subjects contained high concentrations of this polyol (greater than 110 mumol/l), and there was a tendency for the 24-h excretion of it to become more variable in direct proportion to its plasma concentration. In contrast, diabetic patients showed lower plasma concentrations of this polyol, and the variation in the 24-h excretion of 1,5-anhydro-D-glucitol was especially notable among the patients with an extremely low plasma concentration of the polyol. This diabetic group showed a statistically significant correlation (p less than 0.01), between the urinary 1,5-anhydro-D-glucitol and urinary glucose. This correlation was more markedly demonstrated during a 100-g oral glucose tolerance test: parallel changes were observed in the concentrations of 1,5-anhydro-D-glucitol and glucose in the urine collected every hour after the glucose load. These observations led to the proposal that low plasma concentration of this polyol, which is observed in diabetes mellitus, may be the result of a frequent and/or prolonged high blood glucose concentration beyond the renal threshold for glucose excretion.