Dietary fat clearly affects plasma lipid lipoprotein levels. A better understanding of the effect of individual fatty acids on plasma lipoprotein levels has been achieved, although many issues still remain unresolved. Uncertainty with respect to the effects of monounsaturated fatty acids relative to polyunsaturated fatty acids exists. Initial data in primates suggest that dietary fat alters LDL-cholesterol levels via changes in the fractional catabolic and production rates, whereas it alters HDL-cholesterol levels via changes in the fractional catabolic rate.