Rates of dementia in the UK appear to be falling, according to new research.
A study found a reduction from expected prevalence levels of almost a quarter in the last 20 years.
Scientists compared two surveys of randomly chosen people aged 65 and over conducted two decades apart in Cambridgeshire, Newcastle and Nottingham.
Data from questions about lifestyle and health allowed the researchers to estimate national dementia rates in 1991 and in 2011.
From the later findings they calculated that around 670,000 people aged 65 and over now had dementia in the UK. This was 24% lower than the expected figure based on the prevalence in 1991.
Dementia rates remained higher in women, with 7.7 women aged 65 and over affected compared with 4.9% of men.
"This study provides compelling evidence of a reduction in the prevalence of dementia in the older population over two decades," said Professor Carol Brayne, from Cambridge University, who led the research published in The Lancet medical journal.
"Whether or not these gains for the current older population will be borne out in future generations would seem to depend on whether further improvements in primary prevention and effective health care for conditions which increase dementia risk can be achieved."
The term dementia covers a range of conditions that affect thinking and memory. Alzheimer's disease is the most commonly diagnosed form of dementia, with around 417,000 cases in the UK.