With the aging of populaticn, Senile Dementia became an illness of increasing importance in every society, not only as usual in those developed countries but also in developing ones. In order to identify the possible etiology and mechanism of this degenerative and disabling illness, original and review articles about the tio1ogy of Senile Dementia of Alzheimer's type from epidemiologic as well as neurobiochemistric point of view have been examed. From these information at hand, results show that the incidence and prevalence of Senile Dementia are increasing with age, female sex and some social factors. Those factors related to the onset of illness were hereditary and immunologic chracteristics, head injury history and aluminum deposition. Suggestions focuses on a possible mechanism which need furthur epidemiological and neurobiochemistric investigation.