The purpose of this review is to show that urinary angiotensinogen is a marker of early diabetic nephropathy, that oxidative stress increases the expression and secretion of angiotensinogen in the kidney, and that polyphenols with antioxidant effects affect AGT expression in the kidney and to clarify the possibility that soy isoflavones—which have been shown to have strong antioxidant effects—may suppress the progression of diabetic nephropathy through their effects on the expression and secretion of AGT. Here, AGT is linked to diabetic kidney disease.