The nature and distribution of vascular changes in acute Wernicke's encephalopathy (WE) were analyzed in three autopsy cases. Lesions of the lateral vestibular nucleus of the medulla oblongata (three cases) and lateral ventricular wall (one case) were examined by reconstruction of 200 serial sections, and the capillary diameter in the tegmentum of the medulla oblongata was measured morphometrically in all cases. The vascular changes commonly found in and around the parenchymal lesions in all cases were: (1) dilatation and endothelial swelling of almost all vessels ranging from small arteries to veins, being especially severe with undulation in small arteries and arterioles, and (2) fibrinoid degeneration and hemorrhage involving selectively the arterioles and capillaries on the arterial side. These vascular changes in the medulla oblongata were essentially the same as those in the third ventricular wall, but differed in their severity. Capillary diameter in these cases was significantly larger than that in seven control cases. Such vascular changes, especially a dysoric change on the arterial side, could not be ascribed to secondary change following changes in the parenchyma, and were, therefore, considered to be a lesion essential to WE, as in the case of the parenchymal lesion.