The Helsinki Aging Study is based on a random sample of 795 subjects aged 75-years (N = 274), 80-years (N = 266) and 85-years (N = 255). A clinical examination including Clinical Dementia Rating (CDR)-scale was carried out in 82% of the cases. 93 demented subjects were found, 17 of whom had mild dementia. The prevalence of moderate and severe dementia was 2.9%, 10.3% and 23.3% in the age groups of 75-year-olds, 80-year-olds and 85-year-olds, respectively. If we take into account also the mild cases, we get the prevalence of dementia 4.6%, 13.1% and 26.7% in the above mentioned age groups, respectively. The proportion of mild dementias was lower than expected, which probably reflects both the difficulties to recognize mild dementia in an elderly population and the relatively small compensatory capacity of elderly people.