The age-related dementias of the elderly (those aged 65 years or more) are of major public health importance in developed countries. Developing countries, most of which are undergoing epidemiological transition and greying of population, currently contain more than half of the world's population of elderly, a proportion that would reach 75% by 2020. Apart from reports from China, there is little or no information on the dementias of the elderly in developing countries. Alzheimer's disease, which accounts for two-thirds of dementia of the elderly in Caucasian population, is under-documented and believed to be rare in black Africans. But black Americans who are of black African lineage commonly suffer from Alzheimer's disease. A recent autopsy survey of the brains of elderly Nigerians showed absence of senile plaques and neurofibrillary tangles, the pathognomonic histologic lesions of Alzheimer's disease and ageing found in 25% to 80% of normal undemented elderly Caucasians and Japanese. In a community-based door-to-door survey of a population of 9000, including 932 elderly Nigerians, no subject with dementia as defined by DSM-IIIR was found, although there was significant decline of cognition with age, female sex and less than 6 years of formal education. The distribution of cognitive scores is a highly skewed unimodal curve. We emphasize the potential value of cross-cultural epidemiological studies of ethnic groups in different environments and with different prevalence ratios of Alzheimer's disease, in identifying putative environmental factors for this disease.