Although it is known that the severity of dementia in patients with diffuse Lewy body disease is related to cortical Lewy body density, the morphological substrate of dementia in these patients is poorly understood. Vibratome sections processed free-floating for ubiquitin immunohistochemistry in three patients with the common form and one patient with the pure form has shown the presence of large numbers of abnormal, ubiquitinated neurites in the cerebral cortex, mainly cingulate cortex, entorhinal cortex and temporal lobe, and hippocampal complex, regions in which, precisely, Lewy bodies are most abundant. Abnormal neurites are a consistent change which results in abnormal neuronal connectivity. Abnormal cortical neurites, rather than cortical Lewy bodies, may play a significant role in the development and progression of cognitive deficits in patients with diffuse Lewy body disease.