Stroke is the sudden loss of brain function. It is a medical emergency that may result in paralysis, severe brain damage, or death. Most strokes occur when a blood clot blocks the flow of blood to the brain, interrupting the brain's supply of oxygen and nutrients. If the blockage lasts for more than a few minutes, permanent damage occurs. Strokes also may result when a blood vessel ruptures and bleeds into the brain or the fluid around the brain. The bleeding produces pressure that damages brain tissue.

Stroke is a major health problem throughout the world. It is the third leading cause of death in the United States, behind heart disease and cancer. Each year, about 300,000 Americans suffer strokes. About a third of these strokes are fatal. Most people who survive their first stroke soon regain some lost brain function and may regain more over several years. People who suffer two or more strokes are more often permanently disabled. Most stroke victims are age 65 or older.