This review presents a discussion of potential mechanisms by which dietary polyunsaturated fats (including both n-3 and n-6 fatty acids) lower cholesterol concentrations in plasma lipoproteins. The prevailing evidence suggests that the decrease in plasma low-density lipoprotein cholesterol induced by polyunsaturated fat is a result of modified secretion of cholesteryl ester (in the case of n-6 fatty acids) and triglyceride (in the case of n-3 fatty acids) and increased low-density lipoprotein receptor function. These changes protect against atherosclerosis in spite of the concomitant reduction in high-density lipoprotein cholesterol concentrations which appear to be mediated through decreased production and increased catabolism of apolipoprotein A-l, the major apolipoprotein of the high-density lipoprotein.