What’s your blood type? Perhaps you can’t say off the top of your head…and unless you need a transfusion or a transplant, you may not think that your blood type matters much to your health. But recent research suggests otherwise—because it now appears that blood type is linked to your risk for stroke.

Background: There are four major blood groups—A, B, AB and O. Each group is further categorized as positive or negative, depending on the presence or absence of a substance called Rh factor. The new finding is based on data involving nearly 90,000 women and men who were participants in two long-running Harvard University research projects—the Nurses’ Health Study and the Health Professionals Follow-up Study.

Findings: Type O blood was associated with the lowest stroke risk. Compared with people who had type O blood, people with type AB blood were 29% more likely to have an ischemic stroke (the most common type, which is caused by a clot in a blood vessel supplying blood to the brain)…while women (but not men) with type B blood were 17% more likely to have an ischemic stroke. People with type A blood were not significantly different in terms of stroke risk from those with type O. Rh factor was not related to stroke risk.

It is not known why blood type would affect stroke risk. However, previous studies have linked blood type to other cardiovascular risk factors, such as increased cholesterol levels and dysfunction of the blood vessels’ inner lining.

Keep in mind: These results, while intriguing, are preliminary. Even if future research confirms the findings, it is important to recognize that other factors, such as high blood pressure, being overweight and smoking—all of which you can control—boost your odds of suffering a stroke more than your blood type does. While everyone should take steps to control these risk factors, this study provides extra incentive for people with type AB or type B blood. Don’t know your blood type? Ask your doctor.