Glomerular hyperfiltration (GHF) may be an important factor in the initiation of glomerular damage and in predisposing diabetic patients to the later development of diabetic nephropathy (DN). Previous reports show wide range of prevalence of GHF in type 2 diabetic patients. This cross-sectional study was designed to determine the prevalence of GHF at an early stage of DN in Japanese type 2 diabetic patients and to investigate the relationships between clinical variables and GHF. We measured the glomerular filtration rate (GFR) using the plasma clearance of iohexol in 56 control subjects and 93 type 2 diabetic patients without hypertension or overt proteinuria. We used Altman's method to calculate the age-adjusted 95% reference ranges for GFR from the data of control subjects and classified GHF in type 2 diabetics from the reference ranges. Hyperfiltrators (defined as GFR > mean GFR + 1.96 S.D. of control subjects) was found in 17% patients (16/93). Other 77 patients (83%) were normofiltrators (defined as GFR< or = mean GFR + 1.96 S.D. of control subjects). GFR values, both in hyperfiltrators and normofiltrators, were 140.5 +/- 14.6 and 98.8 +/- 14.0 ml/min/1.73 m2, respectively. Age, sex, BMI, blood pressure, albumin excretion rate, and frequency of microalbuminuria did not differ between the hyperfiltrators and normofiltrators. Fasting plasma glucose and hemoglobin A1c were significantly higher in the hyperfiltrators than the normofiltrators (P<0.001 and 0.004, respectively). GHF exists among Japanese type 2 diabetic patients with no evidence of overt proteinuria or hypertension. Glycemic control might be a significant determinant of GHF in these patients.