Diffuse Lewy body disease (DLBD), in its pure form, is a neuropathologic condition in demented patients that is characterized by subcortical and diffusely distributed neocortical Lewy bodies with little or no concomitant Alzheimer's disease (AD) pathology. Clinically, DLBD patients initially present with dementia of insidious onset and subsequently develop mild extrapyramidal motor dysfunction. The present study retrospectively examined the neuropsychological test performance of five patients with DLBD and compared their performance to that of equally demented patients with neuropathologically confirmed "pure" AD. The results showed that the DLBD patients were globally demented with deficits in memory, attention, language, psychomotor performance, and "executive" functions, and a strikingly severe deficit in visuospatial and visuoconstructive abilities. The visuoconstructive and psychomotor impairments of the DLBD patients were significantly worse than those of the AD patients, whereas the memory performance of the AD patients was worse than that of the DLBD patients. These results indicate that DLBD without concomitant AD pathology can produce a global dementia with aspects of both cortical and subcortical dysfunction and suggests that Lewy body pathology contributes importantly to the clinical manifestation of the Lewy body variant of AD.