Neuropsychological syndrome was defined in 94 patients with mixed Alzheimer's/vascular dementia, 75 patients with vascular dementia and 114 patients with Alzheimer's disease (AD) without clinical signs of cerebrovascular disease. Based on this data, the authors singled out main components of neuropsychological disorders in mild and moderate mixed dementia. The comparison of results of the quantitative assessment of neuropsychological functions (total parameters of three blocks of the brain according to A.R. Luria) in mixed dementia and AD revealed significant between-group differences at the stage of mild dementia. No differences was found between AD and moderate dementia. It has been concluded that the assessment of neuropsychological functions should be used for differential diagnosis of different forms of late-onset dementia and may help in understanding of the contribution of risk vascular factors to the development of dementia.