Lewy bodies (LBs) are intracytoplasmic neuronal inclusions sometimes found in the brain stem, diencephalon, basal ganglia, and cerebral cortex. Cases designated as diffuse Lewy body disease (DLBD) demonstrate widespread cortical and subcortical Lewy body formation. The fact that DLBD is possibly the second most common cause of dementia after Alzheimer's disease is not generally recognized. We hope to emphasize the importance of this common neurodegenerative disorder by reviewing the literature and our own experience with DLBD. The English-language literature dealing with the clinical and pathological features of DLBD was reviewed. Pathological material from the Canadian Brain Tissue Bank, Toronto, Ontario, was reviewed over a 2-year period from 1991 through 1993. Prominent LB pathology may occur in isolation or mixed with pathological changes seen in Alzheimer's disease. Lewy body diseases include Parkinson's disease that presents with a classic movement disorder and sometimes dementia, and DLBD where LBs occur in a widespread distribution in the cortex in addition to the usual subcortical sites. Diffuse LB disease usually presents with a neurobehavioral syndrome that may include hallucinations, delusions, and psychosis; all patients eventually become demented. A day-to-day fluctuating mental state may be an important distinguishing clinical feature. Parkinsonism may follow the psychiatric disturbance although occasionally it is a presenting feature. Serious life- threatening side effects may occur with the use of standard neuroleptics. The variable clinical features and additional presence of Alzheimer-type pathological changes in many cases of DLBD has led to a confusing and inconsistent classification of LB disease and, together with little awareness of its existence, its misdiagnosis. Although DLBD may be the second most common cause of dementia, the terminology and classification of LB disorders and their relationship to Alzheimer's disease remain sources of intense debate. Further research is needed to resolve these issues and to provide insight into the pathogenesis of LB formation and accompanying neuronal degeneration.