In cases of Alzheimer's presenile and senile dementia, Alzheimer's disease (AD) and senile dementia of the Alzheimer type (SDAT), respectively, we have observed, in addition to the gray matter degeneration, a lesion that has the character of an incomplete infarction confined to the white matter. It is encountered in 60% of both groups, with mild changes in two thirds and moderate or severe changes in one third. It involves the deep white matter symmetrically, tapering off toward the cortex. It is characterized by partial loss of myelin, axons, and oligodendroglial cells; mild reactive astrocytic gliosis; and sparsely distributed macrophages as well as stenosis resulting from hyaline fibrosis of arterioles and smaller vessels. No complete or cavitating infarctions and no hypertensive vascular changes were observed. The white matter changes are thought to be due to hypoperfusion of the concerned white matter territories since, in addition to the white matter hyaline vascular stenosis, these cases show signs of cardiovascular disease, usually with hypotension. The white matter disorder also occurs independent of the gray matter process of AD and SDAT and may be seen as the sole brain lesion in non-AD subjects. Its occurrence is thus neither regularly related to the severity nor to the regional appearance and accentuation of the cortical Alzheimer process and is thus not likely to be just the result of a wallerian degeneration. Histologically it is similar in several respects to Binswanger's disease, although with some distinct differences. It is thus related to the cerebrovascular group of disorders in addition to AD and SDAT. In view of its frequency and severity, this white matter lesion is important to define, to diagnose, and ultimately to prevent or cure.