The nutritional effects of highly unsaturated fatty acids and oleic acid are discussed. n-6 and n-3 fatty acids are necessary to maintain biological functions but cannot be synthesized in the body. Fatty acids are not only used as energy sources, but also comprise components of biological membranes, which play an important role in information transmission, as precursors of physiologically active materials such as prostaglandins, and as ligands of nuclear receptors. The cholesterol-reducing activity of linoleic acid has been known for decades. The cholesterol-reducing effect of γ-linolenic acid has been reported to be stronger than that of linoleic acid. Some studies concerning the prevention of heart diseases with α-linolenic acid have been reported. An antihypertensive effect of α-linolenic acid has been demonstrated by a large-scale study with Japanese subjects. Numerous epidemiological studies support that the intake of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) is useful for the prevention of cardiovascular diseases. Results concerning the effectiveness of DHA for the prevention and treatment of dementia have also been reported. Research on the nutritional effects of fatty acids must still be developed.