In early type 1 diabetes mellitus, hypertrophy of the kidney is a consistent finding. It is easily diagnosed using current noninvasive methods, especially ultrasonography. Renal functional changes occur in association with hypertrophy, most notably glomerular hyperfiltration. The structural counterpart of this functional change is an early increase in capillary filtration surface area. In most forms of nondiabetic renal hypertrophy, kidney size is closely linked to GFR, but in diabetes, hypertrophy persists after the clinical onset of overt kidney disease (microalbuminuria, hypertension, decreased GFR, etc). The fact that growth factors produced by the kidney can act in both an autocrine and paracrine fashion raises the possibility that the local effects of such substances may act as local mediators of kidney growth, but no such factor has been identified as the initiating or sustaining factor in diabetic hypertrophy. Failure of renal hypertrophy to reverse following strict glycemic control for a few months may turn out to be an important prognostic indicator of future progression of the renal disease, but this remains to be studied in a large group of patients.