Diabetic nephropathy leading to kidney failure is a major complication of both type I (insulin-dependent) and type II (non-insulin-dependent) diabetes mellitus, and glomerular structural lesions (especially expansion of the mesangium) may constitute the principal cause of decline in kidney function experienced by a significant fraction of diabetic patients. Although the biochemical bases of these mesangial abnormalities remain unknown, an understanding of the natural history of diabetic nephropathy from a combined structural and functional approach can lead to greater pathophysiological insight. Work in animals has supported the concept that the metabolic disturbances of diabetes mellitus cause diabetic nephropathy, with structural and functional lesions prevented or reversed with improved or normalized glycemic control. Additional research must address this fundamental issue in humans, especially the response of advancing mesangial lesions to improved glycemic control. Factors not directly related to the metabolic perturbations of diabetes may serve to accelerate or diminish the pathophysiological processes of diabetic nephropathy. The elucidation and management of these factors, when coupled with improved glycemic control, may moderate the development or progression of diabetic kidney lesions in humans.