We have studied 12 patients with diffuse hemorrhagic cerebral amyloid angiopathy clinically and at postmortem examination. The brains in 8 patients had diffuse bilateral loss of myelin in the hemispheric white matter sparing the U fibers, corpus callosum, and internal capsules. The periventricular areas were predominantly affected. Microscopic examination of the white matter showed an association with subacute or chronic edematous lesions: spongiosis, swollen oligodendroglia, widening of the perivascular spaces with edema fluid or siderophages, hyalinization of the blood vessel walls, incomplete myelin loss, and astrocytic gliosis. Three of 8 autopsied patients had undergone computed tomographic examination, which showed bilateral hypodensity of the hemispheric white matter. The brains of 4 patients with illnesses of shorter duration showed only discrete but similar lesions in the centrum semiovale. These white matter changes are similar to those observed in Binswanger's subcortical encephalopathy. We suggest that a common mechanism of hypoperfusion of the distal white matter causes the leukoencephalopathy.