Glomerular filtration rate (GFR), renal plasma flow (RPF) and roentigenographic kiendy size were measured in six newly diagnosed male diabetics with a mean age of 25 years. Glomerular filtration rate was elevated before treatment to the same extent as found previously. A significant fall in both kidney size and glomerular filtration rate was found after treatment with insulin for 3 months. These results support further the concept that there is a basal connection between enlarged kidneys and the elevated GFR of early diabetes.