Hypercholesterolemia, also called high cholesterol, is the presence of high levels of cholesterol in the blood. It is a form of hyperlipidemia, high blood lipids, and hyperlipoproteinemia (elevated levels of lipoproteins in the blood).

Avoiding trans fats and replacing saturated fats in adult diets with polyunsaturated fats are recommended dietary measures to reduce total blood cholesterol and LDL in adults. In people with very high cholesterol (e.g., familial hypercholesterolemia), diet is often not sufficient to achieve the desired lowering of LDL, and lipid-lowering medications are usually required. If necessary, other treatments such as LDL apheresis or even surgery (for particularly severe subtypes of familial hypercholesterolemia) are performed. About 34 million adults in the United States have high blood cholesterol.

Some types of hypercholesterolemia lead to specific physical findings. For example, familial hypercholesterolemia (Type IIa hyperlipoproteinemia) may be associated with xanthelasma palpebrarum (yellowish patches underneath the skin around the eyelids), arcus senilis (white or gray discoloration of the peripheral cornea), and xanthomata (deposition of yellowish cholesterol-rich material) of the tendons, especially of the fingers. Type III hyperlipidemia may be associated with xanthomata of the palms, knees and elbows.

Diet has an effect on blood cholesterol, but the size of this effect varies between individuals. Moreover, when dietary cholesterol intake goes down, production (principally by the liver) typically increases, so that blood cholesterol changes can be modest or even elevated. This compensatory response may explain hypercholesterolemia in anorexia nervosa. A 2016 review found tentative evidence that dietary cholesterol is associated with higher blood cholesterol. Trans fats have been shown to reduce levels of HDL while increasing levels of LDL. LDL and total cholesterol also increases by very high fructose intake.

Classically, hypercholesterolemia was categorized by lipoprotein electrophoresis and the Fredrickson classification. Newer methods, such as "lipoprotein subclass analysis", have offered significant improvements in understanding the connection with atherosclerosis progression and clinical consequences. If the hypercholesterolemia is hereditary (familial hypercholesterolemia), more often a family history of premature, earlier onset atherosclerosis is found.

Increasing soluble fiber consumption has been shown to reduce levels of LDL cholesterol, with each additional gram of soluble fiber reducing LDL by an average of 2.2 mg/dL (0.057 mmol/L). Increasing consumption of whole grains also reduces LDL cholesterol, with whole grain oats being particularly effective. Inclusion of 2 g per day of phytosterols and phytostanols and 10 to 20 g per day of soluble fiber decreases dietary cholesterol absorption. A diet high in fructose can raise LDL cholesterol levels in the blood.

Among people whose life expectancy is relatively short, hypercholesterolemia is not a risk factor for death by any cause including coronary heart disease. Among people older than 70, hypercholesterolemia is not a risk factor for being hospitalized with myocardial infarction or angina. There are also increased risks in people older than 85 in the use of statin drugs. Because of this, medications which lower lipid levels should not be routinely used among people with limited life expectancy.