A case of a 60-year-old man with biopsy-proven diffuse Lewy body disease was presented with clinical and serial neuropsychological data. Findings showed a rapid decline over a 2-year period characterized by an atypical degenerative dementia, including a global mental decline with preserved orientation, profound perceptual deficits, and no specific relative progressive memory impairment. Few extrapyramidal signs were noted, but subtle visual hallucinations and vivid nightmares occurred throughout the course. He underwent an experimental trial of tacrine, but showed an adverse reaction. Of the few detailed neurocognitive case descriptions of this disease found in the literature, this article highlights the unique variability and clinical manifestations that differentiate this disorder from the dementia of the Alzheimer's type.