Age has been reported as a strong risk factor for dementia. Supporting data have been derived mainly from prevalence studies, which had varied criteria and sample compositions that precluded direct comparisons, especially among those aged 85 years and older. Data regarding rates of dementia are presented based on 85 incident cases in the Bronx (NY) Aging Study, a prospective study of 488 initially nondemented, old old persons (mean age on entry, 79 years). Overall, the incidence rate over 8 years of follow-up for all-cause dementia was 3.4 per 100 per year (43% Alzheimer's disease, 30% mixed Alzheimer's and vascular, and 27% other). Incidence rose significantly, irrespective of gender, as subjects were followed up through three age intervals--ages 75 to 79 years (1.3/100 per year), 80 to 84 years (3.5), and 85 years and older (6.0). The comparable age-associated prevalence rates of dementia were 3.7%, 12.2%, and 23.9%, respectively, with an overall period prevalence of 22.8%. Additionally, there was a threefold greater mortality associated with dementia. In conclusion, despite the shortened life expectancy of demented persons, dementia is a highly prevalent condition among those aged 85 years and older. Public policy attention is warranted, since this group is the fastest growing population subgroup.