The observation of an exponential increase in senile dementia prevalence with age has led to the conclusion that this disease may be inevitable in those who live long enough. The alternative view is that at very high ages the prevalence rate levels off. Studies conducted to date have not included sufficient numbers of very old people to resolve this difference of opinion. The question is important both to our understanding of the biological mechanisms involved and for public health planning. We have carried out a meta-analysis of nine epidemiological studies of senile dementia that used DSM III diagnostic criteria and that included samples of elderly people over age 80. The resulting curve was best described as a flattened S curve that fitted a modified logistic function rather than an exponential pattern. The rate of increase in senile dementia prevalence was found to fall in the age range 80-84, and at around the age of 95 prevalence was seen to level off to about 40%. It seems that senile dementia is better conceptualised as an "age-related" (ie, occurring within a specific age range) rather than as an "ageing-related" disorder (that is, caused by the ageing process itself). Very elderly survivors may be at diminishing risk of dementia and this has implications for public health policy.