Background. The pathology of the umbilical arterial endothelium in normal pregnancy and in pregnancy complicated with pre-eclampsia remains unclear. In this study the changes that occur in the umbilical artery endothelial cells were examined and endothelial cell morphology and apoptosis were compared among control, mild, and severe pre-eclamptic subjects. Methods. Umbilical cords with a gestational age of between 35 and 40 weeks were collected from women with normal pregnancies (n = 17), mild pre-eclampsia (n = 10), and severe pre-eclampsia (n = 12). We studied the umbilical artery endothelial cells using flow cytometry, and light and electron microscopy. Apoptosis was measured using flow cytometry and the terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick end labeling technique. The Kruskall-Wallis variance analysis and Mann-Whitney U-tests as post hoc were applied. Results. In mild pre-eclamptics, the endothelial cells appeared ultrastructurally separated. A dilated endoplasmic reticulum, swollen mitochondria, and vanished mitochondrial cristae were observed. In severe pre-eclamptics, the cells were disorganized, highly contracted and vacuolated, separated from each other, and protruding prominently into the lumen. The percentages of endothelial cells that underwent apoptosis in mild (p < 0.017) and severe pre-eclamptics (p < 0.017) were higher than those in the controls. These apoptosis values were highest in severe pre-eclamptics (p < 0.0001). Conclusion. Apoptosis and structural disruptions in the arterial endothelium of severe pre-eclamptics were prominent in all subjects. Increased endothelial apoptosis and structural disruptions are clinically related to intensity of pre-eclampsia, and may be associated with adaptation of the endothelial cells to pre-eclampsia.