The pathological basis for dementia in Lewy body disease (LBD) remains controversial. While some investigators propose that cortical lesions are responsible, others favor a subcortical basis for this dementia. Brains from 35 patients with LBD (11 demented with diffuse LBD; 12 demented and 12 non-demented with a brainstem type of LBD) were clinicopathologically examined to elucidate the pathological differences between demented and non-demented patients with LBD. In cases of diffuse LBD, the cortical lesions were found to be responsible for the dementia. In some of the cases (25%) with the brainstem type of LBD, the dementia was attributed to an Alzheimer pathology, while in many cases (75%), degeneration in the subcortical nuclei, mainly the nucleus basalis of Meynert, played a major role in the dementia.