A blood test can accurately predict the onset of Alzheimer's disease, according to US researchers.
They showed that testing levels of 10 fats in the blood could predict - with 90% accuracy - the risk of the disease coming on in the next three years.
Their findings, published in Nature Medicine, will now be tested in larger clinical trials.
Experts said the results needed to be confirmed, but such a test would be "a real step forward".
The number of people living with dementia stands at 44 million around the globe and is expected to treble by 2050.
The disease silently attacks the brain for more than a decade before any symptoms emerge. Doctors think drug trials are failing because patients are simply being treated too late to make a difference.
This is why discovering a test that predicts the risk of dementia is a major priority for the field.
Scientists at Georgetown University in Washington DC analysed blood samples from 525 people over the age of 70 as part of a five-year study.
They took 53 of them who developed Alzheimer's or mild cognitive impairment and compared their blood with 53 who stayed mentally agile.
They found differences in the levels of 10 lipids, or fats, between the two groups.
And when the research team looked in the other blood samples, those 10 markers of Alzheimer's could predict who was likely to enter mental decline in the following years.
Howard Federoff, professor of neurology at Georgetown University Medical Center, told the BBC: "I think there is a huge need for a test.
"But we must look at larger numbers of people before this could be used in clinical practice."
The full power of the test has not been investigated either. So far they know a diagnosis of dementia can be predicted three years ahead of time, but the researchers are now investigating whether the test works even earlier.
It is not clear exactly what is causing the change in fats in the blood, but it could be a residue of the early changes in the brain.
Dementia across the globe
44 millionglobally have dementia
135 millionwill have the disease in 2050
By then71%will be poor and middle income
$600bnglobal cost of dementia
In the UK, cancer research gets8xas much funding as dementia
A successful test for Alzheimer's could transform medical research and treatment drugs could be tested at a much earlier stage in the disease.
Prof Federoff said slowing the pace of the disease could have a huge impact: "Even a short delay of symptoms will have a tremendous economic benefit just in terms of the cost of care."
Dr Simon Ridley, from the charity Alzheimer's Research UK, said the findings were "encouraging" and that a blood test would be a "real step forward".
He added: "To test the effectiveness of potential new drugs, it's important to be able to recruit people to clinical trials in the early stages of the disease, when such treatments are most likely to be effective.
"If confirmed, these results could also aid efforts to develop better tools for diagnosing Alzheimer's - allowing people with the disease to access crucial support and existing treatments sooner."
The Alzheimer's Society's Dr Doug Brown said the test needed to be investigated further, but could pose ethical challenges.
"If this does develop in the future people must be given a choice about whether they would want to know, and fully understand the implications."