Diabetic nephropathy develops in only a subgroup of patients affected by type 1 diabetes. Once established, this complication is quite difficult to control, and consequently its prevention is presently considered the first target to achieve. As hyperglycemia has immediate effects on renal function and structure, it is reasonable to say that, in predisposed individuals, the pathogenesis of diabetic nephropathy also begins with the onset of diabetes. Two major hypotheses have been put forward to explain how the renal abnormalities that characterize the first years of diabetes - i.e., hyperfiltration and hypertrophy - will later lead to the appearance of microalbuminuria and end-stage renal disease. The first hypothesis considers hyperfiltration as the driving dysfunction and hypertrophy as the consequence. Conversely, the second points to proximal tubule hypertrophy as the starting point and hyperfiltration as the result. Whatever the case, tight glucose control from the very first days of disease seems to be the only established therapy to prevent future complications in type 1 diabetes.