Lewy bodies, the characteristic pathological lesion of substantia nigra neurons 
in Parkinson's disease (PD), are frequently observed to accompany the amyloid 
plaque and neurofibrillary tangle pathology of Alzheimer's disease (AD). However 
the typical anatomic distribution of Lewy bodies in AD is distinct from PD. The 
most common site of occurrence is the amygdala, where Lewy bodies are observed 
in approximately 60% of both sporadic and familial AD. Other common sites of 
occurrence include the periamygdaloid and entorhinal cortex, while neocortical 
and brainstem areas develop Lewy bodies in a lower percentage of cases. In 
contrast, dementia with Lewy bodies (DLB), defined by widespread neocortical and 
brainstem Lewy bodies but frequently accompanied by variable levels of AD-type 
pathology, represents the other end of a spectrum of pathology associated with 
dementia. The observation of Lewy bodies in familial AD cases suggests that like 
neurofibrillary tangles, the formation of Lewy bodies can be induced by the 
pathological state caused by Abeta-amyloid overproduction. The role of Lewy body 
formation in the dysfunction and degeneration of neurons remains unclear. The 
protein alpha-synuclein appears to be an important structural component of Lewy 
bodies, an observation spurred by the discovery of point mutations in the 
alpha-synuclein gene linked to rare cases of autosomal dominant PD. Further 
investigation of alpha-synuclein and its relationship to pathological conditions 
promoting Lewy body formation in AD, PD, and DLB may yield further insight into 
pathogenesis of these diseases.