To investigate whether the increased GFR in diabetic patients is related t an increased ECV, GFR ([51Cr] EDTA clearance) and ECV (distribution volume of [51Cr] EDTA) was determined in 26 insulin-dependent diabetic men aged 17 to 50 years (duration of disease 0.1 to 14 years) and compared to the results in normal men. Mean standard GFR +/- SD (that is, GFR corrected to a body surface area of 1.73 m2) was significantly higher in the diabetics than in 26 age-matched controls (127 +/- 21 ml/min vs. 107 +/- 15 ml/min, P less than 0.001). The same held true for the renal function assessed as the ratio GFR/ECV (0.58 +/- 0.10 hr-1 vs. 0.51 +/- 0.07, P less than 0.05). Regression analysis showed that the relation between ECV and body weight and between ECV and body surface area was the same in the diabetic patients as in 44 controls. The increased GFR but normal ECV indicates that the GFR alteration in diabetic patients reflects a real hyperfunction.