Cholesterol: Cholesterol carried in particles of low density (LDL cholesterol) is referred to as the "bad" cholesterol because elevated levels of LDL cholesterol are associated with an increased risk of coronary heart disease. LDL lipoprotein deposits cholesterol on the artery walls, causing the formation of a hard, thick, substance called cholesterol plaque. Over time, cholesterol plaque causes thickening of the arterial walls and narrowing of the arteries, a process called atherosclerosis.