The number of people living with dementia will triple worldwide by 2050, according to a new report.
There has been a 22% rise in the number of people living with dementia globally since previous estimates three years ago.
Published ahead of next week's G8 summit on dementia being held in London, the research showed 44 million people were now thought to be living with dementia.
This would reach 76 million in 2030 before hitting 135 million by 2050, the report stated.
In 2010, the organisation predicted 115 million people would have dementia by 2050.
The report estimated that for Western Europe alone, dementia cases would double from 7.8 million at present to 16 million in 2050.
Researchers said 32% of people with dementia currently lived in G8 countries (38% high income countries).
This compared to 62% who lived in low and middle income countries.
The report added: "By 2050, the proportion living in G8 countries will have shrunk to 21%, while the proportion living in what are currently low and middle income countries will have increased to 71%."
A report published by the Alzheimer's Society in 2007 showed that 800,000 people in the UK had dementia.
This would soar to 1.7 million people by 2051, it predicted.
Marc Wortmann, executive director at Alzheimer Disease International, said: " It's a global epidemic and it is only getting worse - if we look into the future the numbers of elderly people will rise dramatically.
"It's vital that the World Health Organisation makes dementia a priority, so the world is ready to face this condition."
Jeremy Hughes, chief executive of the Alzheimer's Society, said: " Dementia is fast becoming the biggest health and social care challenge of this generation.
"We must tackle dementia now, for those currently living with the condition across the world and for those millions who will develop dementia in the future.
'Lack of funding means dementia research is falling behind other conditions.
"The G8 is our once-in-a-generation chance to conquer this condition and we must see meaningful action after the talking is over."
A Department of Health spokeswoman said: "We know dementia is a growing worldwide challenge - that is why we are taking action.
"The Prime Minister has personally led the charge with his Dementia Challenge launched last year and we are making good progress.
"But now we are leading a global fightback by bringing the G8 countries together to tackle dementia as a global issue for the first time.
"The G8 summit next week will provide a unique opportunity to make real progress much faster, and re-double international efforts to find effective treatments and cures."