The effect of taking a diet in high oleic acid on the lipid metabolism of guinea pigs and humans was investigated. Diets containing 15% palm oil, high oleic acid sunflower oil (HO), and high linoleic acid safflower oil (HL) were fed to different groups of guinea pigs for four weeks. The plasma cholesterol concentration decreased in both HO and HL groups without any decrease in the high-density lipoprotein (HDL) cholesterol. HO increased the resistance of low-density lipoprotein (LDL) to oxidation. Eleven human volunteers were given an extra 10 g of HO for four weeks. Only a marginal change was observed in the plasma lipid concentration and in the levels of biochemical markers, except for the increase in resistance to LDL oxidation. These results suggest that the high concentration of oleic acid in diet had a lipid-lowering effect similar to that of the polyunsaturated fatty acid-rich diet. The longterm intake of a high-oleic acid diet may therefore help to prevent atherosclerosis in its initial stage.