Normal blood pressure is below 120/80.
Prehypertension is blood pressure that ranges from 120-139 for the top number or 80-89 for the bottom number.
High blood pressure (hypertension) is 140 or higher for the top number, or 90 or higher for the bottom number, or you are taking a blood pressure medication.

The top number is your systolic pressure, which is the force of blood against your arteries as your heart beats. The bottom number is diastolic pressure, which is the pressure on your arteries when your heart relaxes between beats.

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Stroke Risk

Having prehypertension may make you more likely to have a stroke, especially in people younger than 65, a recent study shows.

"If you have prehypertension, and you're under 65, it really matters," says researcher and neurosciences professor Bruce Ovbiagele, MD, of the University of California, San Diego. "Most people who get to 65 already have hypertension."

Ovbiagele's team also found that stroke was more likely for people at the upper end of the prehypertensive range. But even if your blood pressure is not that high, it's wise to take action if your blood pressure is heading in the wrong direction.

"No matter what you consider your break point, any increase in blood pressure is generally a bad thing," says Richard Stein, MD, who directs the exercise, nutrition, and cardiovascular program at New York University's Center for Prevention of Cardiovascular Disease.

"It's causing the heart muscle to beat against a higher pressure, so [the heart] is becoming thicker," Stein says, adding that high blood pressure is also stressful for the kidneys.

Becoming More Common

More than a quarter of U.S. adults -- 28% -- have prehypertension, according to CDC estimates. About 20% of people who have prehypertension will go on to develop hypertension, Ovbiagele says. High blood pressure is the leading cause of stroke and also a major risk factor for heart disease and kidney damage.

Prehypertension and hypertension are on the rise. They've become more common as the U.S. becomes more obese and sedentary, Stein says. You're also more likely to get prehypertension if you have diabetes, high cholesterol, or a family history of high blood pressure, Stein says.

Prehypertension is a particular problem among African-Americans. Researchers have reported that African-Americans with prehypertension develop high blood pressure a year sooner than Caucasians. Although the reasons why are unclear, experts do know that "blood pressure is very difficult to control in African-Americans," Ovbiagele says.

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