A pronounced obliterative microangiopathy of the deep cerebral white matter is one of the cardinal features in classical cases of Binswanger's encephalopathy. We have characterised the alterations taking place in the intima, media and adventitia of obliterated arterial vessels in seven autopsy cases of this encephalopathy. The adventitia of fibrosed vessels showed immunoreactive material indicating a marked deposition of normally occurring collagen types, i.e. I, III and V. Similar deposits occurred in degenerated parts of the media. Two of the cases had, in addition, signs of collagen type VI-immunoreactive material in the adventitia and media. The elastica of arteries was often split and formed multiple layers. The inner part of the blood vessel walls contained immunoreactivity to collagen type IV and laminin, indicating increased amounts of basal lamina components. Using actin immunostaining the fibrosed arterial vessels showed a severe reduction of smooth muscle cells of the media. However, many terminal arterioles presented a marked actin immunostaining, possibly indicating hypertrophy of smooth muscle cells. The endothelial cell layer did not show any changes with regard to expression of glucose transporter 1, factor VIII, Ulex europaeus agglutinin I and CD34. Degeneration of the media associated with depositions of collagens type I, III, IV, V and possibly type VI, as well as other components of extracellular matrix, will jeopardize the regulatory functions of the afflicted vessels. The maintenance of the endothelial lining of the obliterated vessels probably counteracts thrombosis in the vessels.