Heart disease is the leading killer in the U.S., and more than 27 million Americans currently have a cardiac condition. But what is your risk of developing heart disease at some point in your entire life? It might be a lot higher than you think, according to a new paper published online Wednesday in The New England of Medicine.

"We are giving incomplete and misleading risk information if we only focus on the next 10 years of someone's life," Donald Lloyd-Jones, an associate professor at the Northwestern University Feinberg School of Medicine and study co-author, said in a prepared statement, referring to the time span that is often given.

"The majority of adults in the United states who are considered to be at low risk for cardiovascular disease are actually at high risk across their remaining lifespan," the researchers wrote in their paper.

As part of a 50-year-long study, researchers have been following more than a quarter of a million adults in 18 different age cohorts. For the new analysis, researchers looked at key cardiovascular disease risk factors, including blood pressure, cholesterol, diabetes and smoking, as well as the participants' cardiovascular disease status each decade from the ages of 45 to 75 years.

Overall, white men had the highest risk—36 percent—of dying from heart disease or stroke during their lifetime. Black men had a 33 percent chance, and both black and white women had about a 27 percent risk.

Not surprisingly, traditional risk factors played a major role in the statistics. Nonsmokers whose blood pressure was less than 120 over 80 without medication, whose cholesterol less than 180 milligrams (per deciliter) and who did not have diabetes fared the best. Men in this category at age 45 have just a 1.4 percent chance of getting heart disease or having a stroke. When two or more of those factors are not optimal, however, the lifetime risk jumps to nearly 50 percent.

"Just even one small increase in risk," Lloyd-Jones said, "like slightly elevated cholesterol or blood pressure, significantly bumps up a person's lifetime risk." And most people in the study had at least one major risk factor.

The new data will also "help guide public health policy" by giving a clearer picture of the future burdens of these diseases, the researchers noted in their paper. Annual costs of treating heart disease in the U.S. are already more than $272 billion and are projected to exceed $818 billion by 2030, according to a study released last year. It also "underscores the importance of lifestyle—particularly diet, exercise and smoking cessation" in fending off heart disease and stroke in the first place, said co-author Jerry Berry in a prepared statement. Berry is an assistant professor at Texas Southwestern Medical Center.

Lloyd-Jones suggested one way to diminish lifetime risk is to focus on prevention early —that is, to get "kids and young adults off to better starts so they don't gain weight and are following healthier lifestyles throughout their lives."