The surface ultrastructure of uterine arteries from 20 women was studied with scanning electron microscopy. Specimens were excised and fixed under pressure shortely after removal of the uterus during hysterectomy. Two groups of patients were selected: non-smokers and women who smoked more than 15 cigarettes a day. The prevalence of seven different morphological criteria was evaluated without knowledge of the smoking habits of the patients. Endothelial cells were usually well demarcated and distributed with their length axis paralledl to that of the artery. The cell nucleus was revealed by a gentle bulging into the lumen, and cell borders by the presence of rows of microvilli. Inter- and intracellular holes alon the cell borders were more common in arteries from smokers, whereas microvillie were more common on the cell surfaces of arteries from non-smokers. These differences might be related to cell injury. Previous studies indicate that holes are more frequent in injured than in non-injured cells, whereas the surface of injured cells is often smoother than that of non-injured cells. Therefore, these data may be taken to suggest that smoking can lead to endothelial injury in man.