We developed a new method for measuring serum 1,5-anhydroglucitol (1,5-AG), using liquid chromatography/atmospheric pressure chemical-ionization mass spectrometry (LC/MS). With this method we measured serum 1,5-AG concentrations in uremic and diabetic patients and compared these values with those determined by an enzymatic method. Serum 1,5-AG concentrations were significantly less in the undialyzed and dialyzed uremic patients and in diabetic patients; the values in the dialyzed patients decreased after hemodialysis. In uremic patients with high concentrations of serum myo-inositol, the 1,5-AG values determined by LC/MS were significantly lower than those determined enzymatically. There was no significant correlation between 1,5-AG and fructosamine in the uremic patients. These results demonstrate that serum 1,5-AG cannot be used as an index for glycemic control in uremic patients and that the LC/MS method is indicated in uremic patients with high concentrations of myo-inositol.