A new study suggests that as many as 17 million adults in the U.S. may have high blood pressure, despite normal readings at doctor visits.

The study, published in the American Journal of Epidemiology, found that about one in eight adults may have masked hypertension — blood pressure that runs higher when outside of a medical office. Those at highest risk for masked hypertension were men, people older than age 45, and those with prehypertension or diabetes.

“If you fall into one of these groups, it’s worth a discussion with your doctor to talk about other ways to check it,” says Opal Thakar, M.D., a family medicine physician at Jersey Shore University Medical Center. “High blood pressure is a serious heart disease risk.”