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Serum cholesterol is preferably transported by low density lipoproteins (LDL) and high density lipoproteins (HDL). Accordingly, it is generally referred to as LDL-cholesterol and HDL-cholesterol, respectively. In recent years, the vasoprotective role of HDL and the negative correlation between HDL-cholesterol and cardiovascular diseases have been discussed by a large number of authors. This paper discusses the metabolic, pathophysiological, and clinical importance of HDL and HDL-cholesterol from a gerontological point of view. Incorporation of cholesterol into HDL enables cholesterol to be eliminated from the vessels and catabolized in the liver through various mechanism (LCAT substrate, cholesterol esterification and transport, and competitive LDL inhibition). Results of epidemiological, clinical, angiographic, and experimental studies showed that there is a reduction in the concentration of HDL-cholesterol in the case of coronary and peripheral arteriosclerotic diseases. Determination of HDL-cholesterol (in connection with a calculation of LDL-cholesterol) allows the risk of arteriosclerosis to be diagnosed and prognosed, respectively. "Normal" values of HDL-cholesterol are between 35 and 55 mg/dl (0.9-1.4 mmol/l for males and between 45 and 65 mg/dl (1.2-1.7 mmol/l) for females. In women, there is noted a decrease with increasing age. Obesity, diabetes mellitus, hyperlipoproteinemia, high-fat and high-carbohydrate nutrition, and ovulation inhibitors tend to decrease the level of HDL-cholesterol. Physical conditioning, change in diet, reduction of weight, and certain drugs having an effect upon the lipid metabolism tend to raise the HDL-cholesterol level. Knowledge of problems associated with what is here referred to as HDL-cholesterol is of great clinical importance to gerontologists as regards the prevention and therapy of arterio-sclerotic and, more specifically, coronary diseases.