Of two major forms (myo- and chiro-inositol) of inositols, only chiro-inositol enhances the activity of proteins involved in intracellular glucose metabolism. This study aims to determine the urinary myo-/chiro-inositol ratio in type 1 and type 2 diabetes patients and compare its ratio with the normal control group. The 24-hour urinary myo- and chiro-inositols in 71 Korean diabetes patients and 39 control subjects have been quantified using high-performance liquid chromatography, and their ratios have been evaluated as indices of insulin resistance. The level of 24-hour urinary myo-inositol was significantly higher in both type 1 and type 2 diabetes than with the control group, whereas the urinary chiro-inositol in type 1 or type 2 diabetes was lower than that in normal subjects. The myo-/chiro-inositol ratio in diabetes patients was higher than that in the control group. Twenty four-hour urinary myo-/chiro-inositol ratios were significantly elevated in type 1 and type 2 diabetes patients compared to the control group, suggesting that a high ratio of urinary myo-/chiro-inositol in type 2 diabetes patients might be used for an index of insulin resistance.