Objective: To investigate ultrastructural changes in placental bed, and myometrium in preeclamptic women.Methods: This prospective randomized study consisted of 15 severe preeclamptic, 5 normotensive pregnants as study and control groups, respectively. Biopsy specimens were obtained under direct vision at the time of cesarean section. Tissues were obtained from the central placental bed and myometrium, and were examined by electron microscopy.Results: Extensive ultrastructural endothelial damage was found in all the specimens obtained from preeclamptic patients but only one in the normotensive patients.Conclusions: Altered endothelial cell function and endothelial cell injury may be responsible for the characteristic pathology of preeclampsia.