Substantial numbers of cortical and subcortical Lewy bodies are seen in approximately one quarter of patients whose brains show sufficient histopathologic changes for a neuropathologic diagnosis of definite Alzheimer disease (AD). This subset of cases has been named the Lewy body variant of AD (LBV). Despite comparable dementia and the presence of neocortical senile plaques in LBV patients, the overall burden of neuropathologic changes, in particular neurofibrillary tangles (NET), is less than in classic AD. While NFT frequency correlates with dementia severity in classic AD, the cognitive impairment in patients with LBV cannot be completely explained by such changes. Since several studies have suggested a role for synapse loss in relation to dementia severity in classic AD, we decided to investigate the role of synapse loss as a candidate for the cognitive impairment of LBV.