Retinal sheathed vessels from five patients with various vascular diseases were examined by light and electron microscopy. All of these vessels were characterized by a marked increase and disarrangement of collagen fibrils in the media and adventitia. Cytoplasmic processes of glial cells had invaded the adventitia in sheathed vessels and even in unsheathed ones. Thickening and multilamination of the basal laminae were observed in both the sheathed and unsheathed vessels. The lumens of most sheathed vessels were still patent, and the blood cells and endothelial cells of these vessels appeared to be intact. We found no essential differences in the ultrastructure of these vessels in the five patients with various vascular diseases. It is concluded that the increase and disarrangement of collagen fibrils may be mainly related to the ophthalmoscopic appearance of the sheathing and that other factors seem to be of lesser significance.