Dementia with Lewy bodies (DLB) is a common but underdiagnosed dementing disorder. Its criteria were defined in 1996, and revised in 2005. DLB is characterised neuropathologically by widely distributed cortical Lewy bodies (LBs), usually associated with Alzheimer-type pathology. We have re-evaluated the neuropathology of 55 autopsied patients with clinically diagnosed primary degenerative dementia to determine the frequency of DLB in this cohort, which was originally examined when neither the entity of DLB nor its diagnostic criteria had been defined. We also evaluated how discovery of a new entity affects previous diagnoses. Of the 55 brains, 16 (29%) contained LBs. All 16 originally had a neuropathological diagnosis of Alzheimer's disease (AD). 11 (20%) fulfilled the neuropathological criteria for DLB. Three patients had AD with LBs in the brain stem only, and two patients had LBs in the limbic cortex only. Because the criteria and reliable markers for DLB were not available at the time of the autopsies, the diagnosis of DLB had not been possible. The common co-occurrence of AD-type pathology in DLB makes the clinical diagnosis of DLB problematic even today. This study also raises the question of the relative significances of Lewy-related and AD-type pathologies to the development of dementia.