Analysis of the pathology of multi-infarct dementia is best carried out using a two-axis approach. The first axis describes the tissue damage, classified as macroinfarcts, cortical microinfarcts, basal ganglionic lacunes, white matter lacunes, dilated perivascular spaces, diffuse white matter rarefaction, and perivascular edema. The second axis describes the vascular abnormalities, classified as atherosclerosis involving the extracranial or intracranial arteries, arteriolosclerosis, congophilic angiopathy, emboli, and no structural abnormality. Multiple infarcts and white matter rarefaction are commonly seen as a component of Alzheimer disease, in keeping with the development of congophilic angiopathy and possibly other vascular changes in the latter disease. Evidence is presented to support the concept that the white matter rarefaction of Alzheimer disease and aging is associated with perivascular edema, rather than partial infarction.