The research fits with other work tying A, B and AB to more risk of blood clots in the legs and heart attacks. Blood type O also has been tied to an increased risk of bleeding, which implies less chance of clots, the cause of most strokes.

"There's increasing evidence that blood type might influence risk of chronic disease," said one of the study leaders, Dr JoAnn Manson, chief of preventive medicine at Harvard's Brigham and Women's Hospital.

"It's not at the level where we want to alarm people and we want to make that clear. But it's one more element of risk that people would want to know about," and it could give them one more reason to keep blood pressure and cholesterol in line, she said.

The study was presented at an American Heart Association conference. It involved 90,000 men and women in two observational health studies that have gone on for more than 20 years.

Looking at the 2,901 strokes that have occurred and taking into account other things that can cause them, such as high blood pressure, researchers found:

-Men and women with AB had a 26 per cent increased risk of stroke compared to those with type O.

-Women, but not men, with B blood had a 15 per cent greater risk compared to those with O.

What's the explanation?

Blood type depends on proteins on the surface of red blood cells. A pattern of immune system responses forms early in life based on them. Certain blood types may make red cells more likely to clump together and stick to the lining of blood vessels, setting the stage for a blood clot, Manson said.

"You can't change it, and we don't know if it's the blood type per se or other genes that track with it" that actually confers risk, said Dr. Larry Goldstein, director of Duke University's stroke centre.

"There are other things that are more important" than blood type for stroke risk, such as smoking, drinking too much and exercising too little, he said.