Despite the emergence of a number of new classification systems, the diagnosis of cerebrovascular dementia remains controversial. Also controversial is the significance of periventricular and deep white matter alterations (WMA) as seen on magnetic resonance imaging (MRI). To further clarify this issue, MRI scans were used to regroup patients clinically diagnosed with Alzheimer's disease (AD) or subcortical ischemic vascular dementia (IVD) into cohorts presenting with either little versus significant WMA on MRI. These two groups were then compared to demented patients diagnosed with idiopathic Parkinson's disease (PD) using a comprehensive neuropsychological protocol. Neuropsychological assessment failed to distinguish between patients with PD and significant WMA. By contrast, both of these patient groups exhibited disproportionate impairment on tests of executive systems functioning, whereas patients with little WMA showed greater impairment on tests of declarative memory and semantic knowledge. These findings constitute further evidence that the pattern of cognitive impairment associated with significant WMA is distinctly different when compared to AD. These results are discussed within the context of a growing body of literature suggesting that elements of the underlying neuropathologies in AD and IVD are linked. Implications for the diagnosis of dementia are also discussed.