Trans fatty acids (TFAs) are positional isomers of cis fatty acids. TFAs are mainly produced by partial hydrogenation of unsaturated fatty acids and widely found in a variety of foods, including margarine, shortening and frying oil. A small amount of TFAs is also found in ruminant fats and milk. The amount of TFA intake in Western Europe and North America probably ranges from 5 to 15g/day. Epidemiological studies have shown that an excessive consumption of TFAs raised the LDL-cholesterol level and lowered HDL-cholesterol in humans. A high intake of TFAs is therefore thought to be a risk factor of coronary heart disease. On the basis of scientific evidence, the U. S. FDA required labeling of the TFA content for conventional food and dietary supplements on January 1, 2006. TFAs are also restricted their uses in the EU. The TFA intake in Japan is estimated to be approximately 1.6g/day, this amount being markedly lower than that in western countries. Although the low TFA intake in Japan is not thought to be an important issue at present, we should pay attention to an excessive intake of TFAs in the future.