Causes and Natural History of Dyslipidemias

This study will evaluate people with dyslipidemias - disorders that affect the fat content in the blood. Fats, or lipids, such as cholesterol and triglycerides, are carried in the blood in particles called lipoproteins. These particles are involved in causing blood vessel diseases that can lead to conditions like atherosclerosis (hardening of the arteries) or heart attack. Participants will undergo accepted medical tests and procedures to evaluate their condition. Most of the test results are helpful in making a diagnosis and in guiding treatment. People with lipid disorders are eligible for this study. Representative types of patients include those with: * Plasma cholesterol levels greater than 200 mg/dl or less than 120 mg/dl * Plasma LDL-C levels greater than 130 mg/dl or less than 70 mg/dl * Plasma HDL-C levels greater than 70 mg/dl or less than 25 mg/dl * Unusual cholesterol deposits or xanthomas (nodules of lipid deposits on the skin) Children under 2 years of age are excluded from the study. Participants will undergo some or all of the following procedures: \- Plasma evaluation. Apolipoproteins (plasma proteins involved in metabolism of cholesterol, triglycerides, phospholipids, and proteins in the blood) and enzymes involved in lipid metabolism are measured.