New study finds that around 9.19 million people in China had dementia in 2010, compared with 3.68 million 20 years earlier.
In what its authors say is the most detailed study into age-related mental health in China, the paper says prevalence of dementia there is rising far faster than thought and the country is ill-equipped to deal with the problem.
Reporting in the journal The Lancet, a team trawled through 89 academic studies published in English and Chinese between 1990 and 2010.
Their aim was to go beyond previous probes where data was sketchy, and derive estimates on the basis of internationally-recognised diagnoses.
They calculate that in 2010 there were 9.19 million people with dementia in China, of whom 5.69 million had Alzheimer's.
This compares with 3.68 million cases of dementia in 1990, of whom 1.93 million had Alzheimer's.
The 2010 estimate means that China that year had more individuals living with Alzheimer's disease than any other country in the world, says the study.
It says global estimates for this disease may have to be ramped up by at least five million cases, or almost 20 percent.
One of the lead authors, Igor Rudan of the University of Edinburgh Medical School in Scotland, told AFP the dementia rise was partly due to a demographic bulge.
China's population live far longer today than two decades ago, in parallel with the country's rise in prosperity, he said.
"Before, the age for dementia, which is usually over 75, was rarely being reached in low- and middle-income countries... All of a sudden you have an explosion in the older population range, which is reflected in the cases of dementia."
Rudan added, though, that demographics only explained part of the rise in incidence.
He did not rule out the possibility that dementia was being detected and recorded more widely today than in the past.
The paper raised tough questions about China's preparedness, given that western countries are only now beginning to realise the hugely expensive bill for helping people with dementia.
For example, the researchers found that dementia is more prevalent among Chinese women than among men.
This has major implications for health policy, as women in China live far longer than men and comprise up to 75 percent of the population aged 85 years or older.
"Adequate resources should be provided at the national, local, family and individual levels to tackle this growing problem," said researcher Wei Wang of Edith Cowan Medical University in Perth, Australia, and Capital, Medical University in Beijing.
"Public awareness campaigns are needed to counteract common misconceptions about dementia -- including that it is not very common in the Chinese population, that it is a normal part of ageing, or that it is better not to know about it because nothing can be done about it."