Diabetic nephropathy is characterized by the thickening of glomerular basement membrane and the expansion of mesangium. These changes are considered to be caused by an accumulation of the extracellular matrix protein including type IV collagen. The mesangial cells are the cells responsible for the synthesis and metabolism of type IV in the mesangial cells are discussed. The role of advanced glycation end products in the pathogenesis of diabetic nephropathy will be also discussed.