Dementia with Lewy bodies (DLB) and Parkinson's disease (PD) are distinguishable clinically but often not neuropathologically. This study aims to test whether the distribution of cortical Lewy bodies differs in these clinicopathological groups and to develop diagnostic protocols for their differentiation. Brains were obtained at autopsy from cases recruited from prospective clinical studies of dementia or movement disorders. All cases with significant pathologies other than Lewy bodies or plaques were excluded. Cases were categorised into either PD without dementia, DLB (dementia first or within 2 years of disease onset), or PD with a later onset of dementia (PDD). The distribution and density of Lewy bodies and Lewy neurites was determined using antibodies to ubiquitin and alpha-synuclein. Cortical Lewy body densities could not separate cases of DLB from those with PDD. However, semiquantitative thresholds in the parahippocampus could separate demented from non-demented cases with high sensitivity and specificity. Interactions between multiple pathologies were determined using factor analysis. Although many cases had CA2 Lewy neurites, this was not associated with severity or duration of either dementia or parkinsonism. Most DLB cases had significant plaque pathology, and severity and duration of dementia was related to both increasing parahippocampal Lewy body densities and neuritic plaque grade. Weighted kappa statistics revealed that the combination of these pathologies indicated a more severe dementia. These results suggest that dual pathologies cause DLB, and high densities of parahippocampal Lewy bodies indicate dementia regardless of additional pathologies.