Increased glomerular filtration rate (GFR) has been implicated in the development of diabetic nephropathy. Large normal interindividual variations of GFR hamper the diagnosis of renal hemodynamic alterations. We examined renal functional reserve (RFR) in children with type 1 diabetes mellitus to assess whether hyperfiltration occurs. The renal hemodynamic response following dopamine infusion was examined in 51 normoalbuminuric diabetic children (7.7 +/- 3.6 years) with a mean duration of diabetes of 6.2 years and compared them with 34 controls. Mean baseline GFR in diabetic children did not differ from the control population (130.7 +/- 22.9 vs. 124.8 +/- 25 ml/min per 1.73 m(2)), whereas renal plasma flow was significantly lower (463.7 +/- 103.9 vs. 587.2 +/- 105 ml/min per 1.73 m(2), p < 0.001), and filtration fraction was increased (29 +/- 8 vs. 21 +/- 2%, p < 0.001), compared with controls. The mean RFR was lower (p < 0.001) than in control subjects (-0.77 +/- 23 vs. 21 +/- 8 ml/min per 1.73 m(2)). This study documents an increased filtration fraction and reduced or absent RFR in children with type 1 diabetes mellitus in the stage before apparent nephropathy. GFR values were within normal range. Although the reduced RFR and increased filtration fraction indicate the presence of hemodynamic changes, their relevance to the development of hyperfiltration and subsequent diabetic nephropathy remains unknown.