The relationship of dementia with Lewy bodies (DLB) and Parkinson's disease with dementia (PDD) is debated. In DLB, dementia antedates Parkinsonism; in PDD, Parkinsonism antedates dementia. Other than presenting features, diagnostic measures fail to distinguish DLB from PDD. There are few or no pathologic differences between DLB and PDD. In most cases cortical Lewy bodies (LBs) are widespread and there is coexistent Alzheimer type pathology, insufficient to diagnose Alzheimer's disease. Given the predominance of Parkinsonism in PDD, neuronal loss in the substantia nigra is more severe in PDD than DLB. Further clinicopathologic studies are needed to define other pathologic differences between DLB and PDD and to explore the role of neuritic, basal forebrain and striatal pathology in these clinical syndromes.