Glomerular hyperfiltration found in diabetic patients is recognized as an early renal alteration and reversible stage of kidney damage, but details have not yet been studied. We investigated the relation among estimated glomerular filtration rate (eGFR) and many factors such as age, HbA1cvalue, blood urea nitrogen (BUN) concentration, Na and K concentrations in serum of Japanese diabetic patients to investigate the characteristics of hyperfiltration [1]. Hyperfiltration (eGFR ≥ 120 ml/ mini/1.73 m2) was found in six among 60 diabetic patients investigated, and Na concentrations in five patients with hyperfiltration were slightly lower than the reference range of Na concentration (138-145 nmol/L), whereas their K concentrations were within the reference range (3.6- 4.9 nmol/L), but significantly lower than those of diabetic patients with normal- and hypofiltration. In contrast, the Na and K concentrations in more than half of patients with normal-and hypofiltration have low Na concentration and high K concentration as compared with these reference ranges. Thus, mild hyponatremia with a relatively lower concentration of K seems to be the characteristic symptoms of hyperfiltration among diabetic patients [2]. BUN concentrations in five patients with hyperfiltration were normal level (below 20 mg/dL), whereas those in the patients with severe hypofiltration (eGFR < 30 ml/ mini/1.73 m2, n=4) were higher than 50 mg/dL. Thus, renal dysfunction estimated by BUN concentration could not be seen in most patients with hyperfiltration, whereas the patients with severe hypofiltration are thought to have chronic kidney disease [3]. No correlation was found between eGFR and HbA1c values of the patients with hyperfiltration. These results are consistent with previous findings that renal dysfunction of diabetic patients with hyperfiltration is mild, an early stage and recoverable.