Nephropathy remains a major cause of morbidity and mortality in diabetes mellitus and increasing numbers of patients require dialysis maintenance therapy. Diagnostic criteria of diabetic nephropathy are well established, but the relationship between the morphological changes of glomerular basement membrane thickening and increased mesangial volume and functional abnormalities, such as proteinuria and impaired renal function, are still not clearly understood. Tissue studies in patients with micro-albuminuria, as well as increased knowledge of the structure of the glomerular basement membrane, should lead to an understanding of how the diabetic process affects blood vessels throughout the body.