We will introduce the differences between PDD and DLB from a neuropsychological perspective. We also will introduce diagnostic criteria for PDD and PD–MCI (PD with mild cognitive impairment), as well as an attempt to detect the onset of dementia at an early stage by examination of dysarthria and olfactory dysfunction. DLB had impairment in memory, attention/executive function, and visual–spatial cognition. Compared to AD, visual–spatial cognition, attention, and working memory deficits are more severe than verbal memory. Comparing cognitive function between DLB and PDD, executive function is more impaired in PDD than in DLB. Among the PD–MCI group, those with impairments in multiple cognitive domains had a high rate of conversion to PDD. DLB has more diverse and severe neuropsychiatric symptoms than PDD. Lewy bodies and neurofibrillary tangles in the limbic system and cerebral cortex are associated with the onset of visual hallucinations, visual misperceptions, and delusions of misidentification. The onset of dysarthria in PD may be one of the symptoms suggesting cognitive decline. It is known that cross–modality integration related to the olfactory and vision in PD is impaired before a cognitive decline occurs and may be related to dopaminergic dysfunction.