Experts at Harvard Medical School in Boston found that light coffee drinkers who normally have no more than one a day are twice as likely to suffer a blood clot on the brain if they increase that by an extra cup or two.
The risk of a potentially fatal stroke was greatest in the hour following consumption. After two hours, the coffee's effects had worn off and the risk of a stroke passed.
But the danger lies in occasional exposure to relatively higher caffeine levels, the findings said - making light coffee drinkers more susceptible.
This could be because coffee-lovers who get through several cups a day become ''desensitised'' to the effects of caffeine, such as raised blood pressure, stiffening of the arteries and higher norepinephrine levels, a stress hormone that increases heart rate.
Researchers examined 400 stroke victims, comparing each one's coffee intake in the hour before their stroke with their usual consumption over the previous year.
Around one in ten had drunk coffee less than an hour before falling ill.
"Consumption was linked with a risk of stroke in the subsequent hour twice as high as during the periods where there was no coffee consumption," the Daily Mail quoted the authors as saying.
They found no such association with caffeinated tea or cola. The risk is mainly confined to those who exceed their normal intake of a cup or so a day.
The study appears in the journal Neurology .