Acute atherosis of the myometrial segments of the uteroplacental arteries from pre-eclamptic pregnancies was studied by electron microscopy. The lesions in their early stages are characterized by endothelial damage, insudation of plasma constituents into the vessel wall, proliferation of myointimal cells, and medial necrosis. Fat acculumation is seen first in myointimal cells and, later, macrophages engulf the lipid-rich debris released from disintegrating myogenic foam cells. Gross endothelial damage, massive intramural fibrin deposition, luminal thrombosis, and vessel rupture with hemorrhage are epiphenomena.