High levels of an amino acid in the blood are associated with an increased risk of stroke, say researchers.
Brain scans can help diagnose a stroke
Doctors have suspected that homocysteine is linked to stroke, but there has been no conclusive evidence.
The London-based team looked at people genetically prone to high homocysteine levels and found they had a higher stroke risk than other individuals.
The good news is that research suggests the risk can be reversed by taking folic acid, they told the Lancet.
But they said large trials were still needed to test whether such treatment would be safe in the long term.
Dr Pankaj Sharma, co-author of the study and consultant neurologist at Hammersmith Hospitals NHS Trust, said: "We are awaiting the results of several large randomised trials to confirm that this approach is effective and safe."
A recent study in the British Medical Journal suggested women who take folic acid supplements late into pregnancy may be at increased risk of breast cancer.
It is believed that too much folic acid can impact on a person's levels of vitamin B12.
It is thought to make it harder to spot a deficiency of vitamin B12, which can lead to neurological damage.
The current study's authors said vitamin B supplements might be needed along with folic acid.
They also pointed out that the risk posed by high homocysteine levels was much smaller than other classic stroke risk factors, such as smoking and high blood pressure.
Therefore, it is unclear whether routinely checking people's homocysteine levels would be beneficial, they said.
Professor Peter Weissberg of the British Heart Foundation said: "We've known there is a link between blood homocysteine levels and stroke for many years, but research in this area has been complicated as homocysteine can be difficult to manipulate.
"Promisingly, this has revealed a much clearer relationship between homocysteine and stroke, suggesting that high levels may themselves cause stroke. We are absolutely delighted to have supported the study."
A spokeswoman from the Stroke Association said: "This is interesting research which adds to the overall knowledge gained on the role of homocysteine as a risk factor for stroke.
"As yet there is no conclusive study which shows that taking folic acid is beneficial for everyone.
"A healthy balanced diet with at least five portions of fruit and vegetables daily will help protect people from the risk of stroke.
"The Stroke Association is watching the progress of the trials with interest and welcomes any future research that will help to reduce the 130,000 strokes that occur each year."