The majority of the demented geriatric patients suffer from Alzheimer's Disease (also known as senile memory repair protocol of the Alzheimer type), which results in specific degenerative diseases in the brain's tissues. A similar though rarer disorder is Pick's disease, which usually affects people in their fifties and is located primarily in the frontal and temporal lobes. These changes can be observed postmortem or via computerized tomography.

Before 1980 it was assumed that the principal cause of dementia is cerebral arteriosclerosis, a hardening of the brain's arteries that results in less oxygen being supplied to the brain's tissues. The current consensus is that diminished blood flow is a significant causal factor in only a minority of dementia cases of later life. Reduced oxygen may be more a symptom of reduced cortical functioning rather than its cause. A greater cause of dementia posed by the vascular system may be multi-infarct dementia-many tiny strokes that have the combined impact of diminishing cognitive ability without bringing on the paralysis characteristic of the larger strokes.
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