Alzheimer's disease (AD) and Parkinson's disease (PD) are generally considered to be separate and distinct disease entities. However, a considerable amount of evidence demonstrates that these disorders share common clinical and neuropathologic features and that overlap between the two conditions is extensive. For example, a significant percentage of AD patients exhibit extrapyramidal features, and many PD patients develop dementia. Similarly, at autopsy many AD patients not only exhibit the neuropathologic features of that disorder but also exhibit nigral pathology, including Lewy bodies. The vast majority of demented PD patients show widespread neurofibrillary tangles and senile plaques as well as Lewy body formation and nigral degeneration. The extent of such overlap is far greater than one would anticipate by chance alone. We argue that such overlap reflects a common pathogenic mechanism for the neurodegeneration encountered within specific vulnerable neuronal populations. Furthermore, we suggest that the current nosologic approach, which attempts to separate AD from PD, fails to properly deal with the issue of overlap and that a new classification of the neurodegenerative disorders should be considered.