To evaluate the role of renal haemodynamic factors in the pathophysiology of diabetic nephropathy, we determined by radionuclear techniques glomerular filtration rate (GFR) and renal plasma flow (RPF) in 18 patients affected by insulin dependent diabetes mellitus (IDDM) in good metabolic control, with normal blood pressure and plasma creatinine. GFR and RPF measured in the same patients after ten months correlated with proteinuria and duration of diabetes. Our finding of a significant correlation between the decline of RPF and duration of diabetes may support the haemodynamic hypothesis of progression of diabetic nephropathy.