Most necropsy surveys of dementia have found that vascular disease is second only to Alzheimer's disease as a cause of dementia. Alzheimer's disease and cerebrovascular disease also often coexist. The purpose of the present study was to determine the nature of the cerebrovascular lesions that are most significant in producing dementia. These were analyzed in a group of cases of dementia in which only vascular pathology was present and, in particular, no more than trivial amounts of Alzheimer-type pathology were present. The cerebrovascular lesions in this group of cases were compared with those in a group of stroke cases who were nondemented and a group of elderly cases without stroke or dementia. Severe cribriform change and deep white/grey matter micro-infarcts were significantly more common in the test group than in either of the control groups, whereas single macroscopic infarcts were more common in the stroke control group than either of the other two groups. Thus, microvascular deep white and grey matter lesions, but not macroscopic infarction, were significant in vascular dementia. The results of this study will be discussed in relation to the view that microvascular lesions may also contribute to dementia in subjects with more extensive Alzheimer-type pathology and thus lower the threshold at which Alzheimer-type pathology becomes clinically manifest.