LDL, which stands for Low-density lipoprotein is considered to be "bad" cholesterol because these molecules transport cholesterol from the liver to all the cells of the body. As it does so, LDL sticks to arterial walls. These fatty plaques can harden into calcified deposits that block the flow of blood and the oxygen it carries to the heart muscle-- not a good thing. LDL is thus used as a predictor of a person's risk of heart disease, because high levels of LDL are associated with a high risk of atherosclerosis, but research indicates that size matters: smaller, denser LDL particles correlate better with increased risk of heart disease.