We describe what we believe to be the fifth case of a degenerative condition of the brain characterized by unusual intracytoplasmic inclusions in neocortical astrocytes, and we review four previous reports of what appears to be the same condition. Whereas these previous cases were characterized by prolonged clinical mental and psychomotor retardation, our case describes a rapid onset in a previously fit and mentally able patient in whom the astrocytic inclusions showed a close association with fibrohyaline vascular degeneration and changes resembling those of Alzheimer's disease. The inclusions, which were most frequent in the second to fourth layers of the frontal, temporal, and occipital cortices but absent from subcortical regions, consisted of large, irregular hyaline bodies surrounding the nucleus and extending into the proximal parts of cell processes. Ultrastructurally they consisted of free ribosomes in a granular and filamentous matrix. They were not bound by a membrane. Lipofuscin granules were associated with them. It is suggested that the inclusions might result from a disturbance of protein metabolism in protoplasmic astrocytes, but their true significance is unknown.