Increased levels of IDL and small, dense LDL are associated with the risk of coronary artery disease. Possible mechanisms include increased susceptibility of small, dense LDL to oxidation, and to other pathologic effects, such as increased retention in the arterial wall. Beneficial effects of a low-fat diet and certain lipid-lowering therapies on the levels and properties of small, dense LDL or their precursors may contribute substantially to the reductions in coronary atherosclerosis observed in several lipid-lowering trials.