(FORTUNE Magazine) – In view of what's happening in the world these days, it's a wonder everyone doesn't have high blood pressure. Maybe they do. More than a quarter of the people in the U.S. with high blood pressure, or hypertension, aren't aware of it, and three-quarters of those who know they have it don't have their pressure adequately controlled. A study published in JAMA in February suggests that someone now age 55 has a 90% risk of developing hypertension by the time he or she reaches 80.

Hypertension is a major risk factor for coronary heart disease, heart failure, stroke, dementia, and kidney failure. You would assume that something this potentially lethal would sound alarm bells in the body, but there are almost never any symptoms.

Blood pressure is determined by taking a reading of the blood flowing through the arteries and expressing the findings in millimeters of mercury. It consists of two numbers. The lower number (diastolic) is the baseline pressure present at all times; it's like the steady pressure of water flowing through a garden hose. The top number (systolic) is the surge of pressure after each pump of the heart. The definition of hypertension is either a systolic reading above 140 or a diastolic above 90. Optimal would be less than 120/80, and anything above 130/85 is considered high-normal.

People with high-normal pressure are at greater risk of developing cardiovascular disease than people with optimal blood pressure. So as with cholesterol--the lower, the better.

Blood pressure increases with age; obesity, alcohol, dietary salt, certain medications, and a sedentary lifestyle are contributing factors. Over 90% of hypertension is called "essential," which means there are no obvious underlying causes.

Since there are no symptoms, people often underestimate the damage high blood pressure can do, and ignore it. Therein lies the problem. The first sign of trouble may be a stroke. Everyone should have his or her blood pressure checked at least every two years, and it doesn't hurt to check it more often.

When I find a patient's blood pressure to be high or even high-normal, I ask him to check it at least two or three times a week to determine patterns in the daily environment. It's easy enough to get a home blood-pressure machine at a drugstore. They are relatively inexpensive, easy to operate, and usually reliable.

If the readings continue to be high-normal or greater, I walk my patients through the lifestyle modifications that can turn the tide: weight loss, regular exercise, decreased intake of salt and alcohol, and the DASH (dietary approaches to stop hypertension) diet, which consists of lots of fruits and vegetables along with low-fat dairy products.

If lifestyle changes don't drop blood pressure below 140/90, the next step is medication. Many different drugs do the job, among them diuretics, beta blockers, ACE inhibitors, and angiotensin receptor blockers. We try to tailor the drug to the individual. For example, the best blood pressure medication for people with diabetes is an ACE inhibitor, because it also helps the kidneys.

If someone experiences side effects from a particular drug, we'll try another. Sometimes it takes three or four tries--or a combination--but it's worth it. Patients live longer and better if their pressure is under control--i.e., less than 140/90.

There are a few numbers everyone should know: Social Security, wedding anniversary, cholesterol, and blood pressure. Knowing them and treating them appropriately will help keep you out of trouble with the government, happy at home, and healthy for a long time.

Donald D. Hensrud, M.D. is director of the Mayo Clinic Executive Health Program. For more information on this topic, go to mayoclinic.com. Mayo Clinic offers Executive Health Programs in Jacksonville, Fla.; Rochester, Minn.; and Scottsdale, Ariz.