Senile dementia of the Alzheimer type is becoming one of the most common of the malignant diseases as our society ages. Currently, research has identified several pathophysiological changes, including the bihelical filament and the loss of the enzyme choline acetyltransferase from the cortex. Although genetic factors play some role in this disease, the important environmental risk factors have not yet been identified and there is, at present, no specific treatment. The second most common cause of dementia, cerebrovascular disease, produces dementia only when there is destruction of brain tissue, as in individuals who have multiple strokes or who have hypertensive vascular disease leading to multiple lacunae. In both multi-infarct dementia and in the lacunar state, hypertension appears to play a greater role than it does in other forms of vascular disease. Many of the other causes of dementia, including normal pressure hydrocephalus, CNS infections or tumors, metabolic disorders produced by thiamine or vitamin B12 deficiency or thyroid dysfunction, are often reversible. Every patient, whatever the age, with a developing dementia deserves a thorough workup to identify these treatable disorders.