The direct relationship between glomerular filtration surface and glomerulaf filtration rate was studied in a group of eight short term, young diabetics exhibiting a 50 per cent elevated glomerular filtration rate with a 2-fold variation within the group. A close correlation was obtained between glomerular filtration surface and glomerular filtration rate, r = 0.8. This supports the idea that the increase in glomerular capillary surface may be a main factor in diabetic kidney hyperfunction. The possible long term implications of the functional and morphologic abnormalities are discussed.