Structural white matter changes were found to be common in dementia of Alzheimer's type. A neuropathological investigation was undertaken in order to characterize and quantify these white matter changes. The tissue changes were compared with those in white matter in normal control cases and in complete white matter infarcts. Our results support the hypothesis that the white matter lesion seen in dementia of Alzheimer's type is due to incomplete infarction involving both axons and myelin and is a result of mild ischaemia. In mild ischaemia, the myelin compartment appears to be the most vulnerable. We have termed the lesion selective incomplete white matter infarction.