As blood circulates, it exerts pressure on the walls of the arteries. Hypertension, or high blood pressure, is characterized by a persistent increase in this pressure, even when a person is at rest. The disorder is one of the most common chronic health problems in the United States, affecting nearly 74 million Americans. High blood pressure is also the most important risk factor for stroke—the third leading cause of death in the United States and the leading cause of disability. If left untreated, this “silent killer,” as high blood pressure is often called, is a primary cause not only of stroke, but also of coronary heart disease, heart failure, kidney disease, and blindness.