The number of people living with dementia is levelling off in parts of western Europe, a report says.
The University of Cambridge study shows the proportion of elderly people with the condition in the UK has fallen, contrary to predictions that cases would soar.
Improvements in health and levels of education might be protecting people from the disease, the scientists said.
Charities warned there was no guarantee the improvements would continue.
The report, in the Lancet medical journal, analysed twinned dementia studies that were conducted in the same way, but decades apart.
Data from five studies from the Netherlands, UK, Spain and Sweden showed that the proportion of people with the condition had stabilised over the periods covered by the studies - which ranged from nearly 20 years to almost 30. But in the UK and among Spanish men, it had fallen.
In the UK, the data from 1991 suggested that 8% of over-65s would have dementia in 2011, yet the team in Cambridge said the figure was in fact 6%.
It means there are around 670,000 over-65s with the condition rather than the 810,000 figure regularly cited.
An ageing population should have led to more people living with dementia. However, lead researcher Prof Carol Brayne said the expected rise "had not occurred".
She told the BBC News website: "Effectively it has stabilised rather than gone up.
"The age-specific prevalence has gone down so even though the population has got older, the number [of patients with dementia] has stayed the same."
She said the findings suggested there may be a "preventable component within individuals, and across whole populations".
"It may be possible that we can defer dementia rather than prevent it entirely - it's very unlikely that we can prevent it entirely."
The exact reason why rates have fallen is uncertain, but improvements to the health of nations are the most likely.
Risk factors for dementia include:
- vascular disease
- little exercise
Some of the people in the earlier studies would have lived through world wars, the Spanish Civil War or the Dutch famine.
The report said heart health and levels of education had changed hugely since then.
The Alzheimer's Society said its figure of 850,000 people with the condition was based on 60 studies.
However, it said there was a "new and emerging" picture showing that dementia might not be increasing as rapidly as previously thought.
It said the condition was already a huge social and economic problem, with a quarter of hospital beds filled with dementia patients.
The charity's chief executive, Jeremy Hughes, said: "With no cure, few effective treatments and an economic impact exceeding that of cancer or heart disease, dementia remains the most critical health and social care challenge facing the UK."
It still predicts there will be one million patients with the condition by 2025, but Prof Brayne said that when it came to future projections, "it's a guess, effectively", as it was uncertain whether people's health would continue to improve and reduce the likelihood of dementia.
Meanwhile, Dr Matthew Norton, from Alzheimer's Research UK, said the findings were encouraging, but the risk factors for dementia were "not yet fully understood".
He also warned: "Current trends in risk factors such as obesity and diabetes mean we should not be complacent. But measures to help people adopt healthy lifestyles now could have a real impact on the numbers of people living with dementia in the future."
Prof Brayne concluded: "Dementia is still common in the older age group. It still doubles every five years after 65.
"What we're hoping from this research is that it will provide more evidence for focusing research beyond drug discovery."