Low-density lipoprotein is known as "bad" cholesterol. When too much LDL circulates in the blood, it can slowly build up in the inner walls of the arteries that feed the heart and brain. Together with other substances, it can form plaque, a thick, hard deposit that can narrow the arteries and make them less flexible. This condition is known as atherosclerosis. If a clot forms from the rupture of plaque in the wall of a blood vessel, the clot can blocks or narrow blood flow in the artery and cause heart attack or stroke.