Atherosclerosis is a slowly progressive disease characterized by the accumulation of cholesterol within the vessel wall. Plasma lipoproteins are particles of complex lipid and protein composition that transport lipids in blood. Low density lipoproteins (LDL) and high density lipoproteins (HDL) are the major cholesterol-carrier lipoproteins. LDL seem to be responsible for the delivery of lipids (cholesterol) from the liver to the tissues. Compelling evidence supports the concept that the lipids deposited in the arteriosclerotic lesions are derived primarily from plasma LDL. The term "reverse cholesterol transport" describes the transport of cholesterol from extrahepatic tissues to the liver, where it may be metabolized. Reverse cholesterol transport seems to be the major route for removal of the exchangeable cholesterol deposited in the extrahepatic tissue. It has been postulated that a major role of the plasma HDL particle is to act as a scavenger of tissue cholesterol. The hypothesis that high levels of plasma HDL are protective against coronary artery disease (CAD) was initially proposed by Barr et al in the early 1950s, but it was overlooked until confirmed by the Tromso and Framingham studies in 1977, which showed an inverse relation between HDL plasma levels and incidence of CAD. Similar observations have been made in 15 major experimental studies including eight countries.(ABSTRACT TRUNCATED AT 250 WORDS)