The ultrastructure of human umbilical arteries from 9 smokers and 14 non-smokers were studied. A double blind technique was used to separate the two groups. The study revealed the following morphological characteristics: in non-smokers presence of mononuclear cells beneath the endothelial lining; in smokers a marked distension of the rough endoplasmic reticulum and a marked increase in number of mitochondria were found in the endothelial cells. The smooth muscle cells had accumulated glycogen and lipid. In the arteries of the smokers there were morphological signs which could be interpreted as an increased cell turn-over within both the intima and media; increased basement membrane material and increased amount of glycosaminoglycans were found; also increased number of lysosomes, and phagocytosis. Smooth muscle cells were seen covering the luminal surface of the vessel wall.