OBJECTIVE An increase in glomerular filtration rate (GFR) and renal plasma flow (EFPR) may be considered as prognostic factors for the progression of diabetic nephropathy; however the real predicting value of hyperfiltration in the development of incipient and overt nephropathy is as yet unknown. We have examined the prevalence of hyperfiltration in a population of normotensive adult IDDM patients and the possible effect of long-term metabolic control on glomerular hemodynamics. METHODS We measured GFR and ERPF values in 177 normotensive, normoalbuminuric insulin-dependent diabetic patients and in 30 healthy subjects by single bolus intravenous injection of 1 miroCu/kg [51Cr]-EDTA and 0.2 microCu/kg [125I]-Hippuran intravenously. We have correlated the GFR values with parameters of metabolic control over the last 3 years and with age, sex, and duration of diabetes. RESULTS Patients with a GFR greater than the 95 degrees percentile value of controls (135 ml/min/1,73 m2) were defined as hyperfiltering. They represented the 55.9% (99/177) of our population. We found a strong correlation between GFR and ERPF (p <0.001), and between GFR and average HbA1c levels (p = 0.016) in multiple regression analysis, with age, sex, ERPF, and average HbA1c levels entered as variables (r2 = 0.45). There appeared to be no correlation with the duration of the disease. CONCLUSIONS Long-term hyperglycemia provides a significant contribution in GFR and a poor metabolic control is predictive of overt nephropathy. In this study hyperfiltration does not appear to be the major factor of diabetic nephropathy. A follow-up of these patients is necessary to clarify the role of hyperfiltration in the development of overt nephropathy in diabetes.