Context: Lewy bodies (LBs) are intraneuronal inclusions in the brain that have been increasingly recognized as neuropathological lesions with relevance not only to Parkinson disease but also to Alzheimer disease. However, the degree to which the density of LBs in the brain contributes to the severity of dementia has not been clear. Objective: To determine the degree to which LB 'burden' contributes to dementia. Design: Brain specimens were examined from 273 consecutive autopsies of elderly subjects residing in a nursing home. The numbers and densities of LBs were determined in multiple brain regions, and their correlation with a measure of cognition and functional status (Clinical Dementia Rating) during the 6 months preceding death was determined. Setting and Patients: Postmortem study of nursing home residents. Results: The severity of dementia correlated significantly and positively with the density of LBs. These correlations were independent of other neuropathological disorders commonly associated with dementia, including Alzheimer disease. The density of LBs correlated significantly with dementia severity whether or not the diagnostic criteria for Alzheimer disease were met and after the contribution of classical Alzheimer disease lesions, neuritic plaques, and neurofibrillary tangles had been accounted for by partial correlation analysis. Concluslon: Lewy body inclusions appear to contribute significantly to cognitive deficits in the elderly in a manner that is independent of other neuropathological disorders.