The prevalence of age-associated dementia which already constitutes enormous public health problems in many developed industrialised countries, is currently low in many developing countries. Community-based studies and autopsy survey in Nigerian Africans showed absence of Alzheimer's disease (AD). The ageing revolution with enormous increase in the population of the elderly is already occurring in developing countries which are also undergoing epidemiological transition. Geographical epidemiological studies of the age associated dementias, in developing and developed countries, and especially in ethnic groups who live in different environments have great potential in identifying putative risk factors, probably currently present in developed industrialised countries and absent in developing less industrialised countries where the disease is as of now relatively uncommon.