The effect of diabetic metabolism on the human vascular wall was studied, using the fetal cardiovascular system at birth as an experimental model. The ultrastructure of umbilical arteries from nine newborn children of nonsmoking diabetic mothers (White group D) was compared with that of 30 healthy nonsmokers. Intimal cushions, thickening of the basement membrane often with a multilaminal appearance, and glycogen accumulations, both in the cells of the intima and the media, were found. The cells of the intima were very rich in fibrillas, identical to the underlying media myocytes. Endothelial cell death with formation of a pseudoendothelium due to migrating myocytes might be the explanation.Circ Res 47: 620-626, 1980