Hypertension is high blood pressure. It is important because it makes a person more likely to develop atherosclerosis, a thickening of blood vessel walls that can lead to heart attacks and strokes. People with atherosclerosis sometimes accumulate calcium in their blood vessel walls. A special form of computed tomography (CT scan) can detect calcium in the blood vessels of the heart (coronary arteries). People with lots of calcium in their coronary arteries are more likely to have heart attacks and strokes. Recently, researchers have recognized that blood pressure levels lower than those usually considered abnormal might still be harmful (prehypertension). The effect of prehypertension on risk for heart attacks and strokes is not entirely clear. Showing that prehypertension contributes to coronary artery calcium would suggest that it is harmful. It would also suggest that the line doctors draw to distinguish harmful from safe blood pressure levels might be lowered.