To investigate the relation between blood glucose control on the one hand and an increased glomerular filtration rate and enlarged kidneys on the other, we studied 12 patients with insulin-dependent diabetes and an increased glomerular filtration rate for a year after they were randomly assigned either to continuous subcutaneous insulin infusion or to unchanged conventional therapy. Glycemic control, measured by mean plasma concentrations of glucose and glycosylated hemoglobin, was rapidly and significantly improved (P less than 0.001) in the pump group but did not change in the conventional-treatment group. In the pump group, the glomerular filtration rate fell significantly in the study period (P less than 0.001) and became normal in four of the six patients. It did not change in the conventional-treatment group. There was no change in kidney volume in either group. At the end of a year, a return to conventional insulin treatment in the pump group resulted in both metabolic deterioration and a significant rise in the mean glomerular filtration rate toward base-line values. We conclude that in patients with established insulin-dependent diabetes, strict glycemic control normalizes the glomerular filtration rate, although the kidneys may remain enlarged.