Dietary fat is known to affect serum concentrations of total and lipoprotein cholesterol. However, all components of dietary triglycerides--saturated, monounsaturated, and polyunsaturated fatty acids--do not have identical effects on serum cholesterol levels. Until recently, most attention has been given to saturated fatty acids, which raise cholesterol levels, and polyunsaturated fatty acids, which are thought by many to lower cholesterol levels. Monounsaturates in contrast have been given little attention. However, recent studies carried out in our laboratory and in others have shown that monounsaturates can have favorable effects when substituted for saturated fatty acids in the diet. In this exchange, the monounsaturates reduce low density lipoprotein (LDL) cholesterol levels, but do not lower high density lipoprotein (HDL) cholesterol levels. In contrast, an HDL-lowering action has been noted for polyunsaturates. Also, monounsaturates appear to alter lipoproteins more favorably than carbohydrates, which can raise triglycerides and lower HDL cholesterol levels. Therefore, monounsaturated fatty acids appear to have more potential for use in cholesterol-lowering diets than previously recognized.