In the process of converting vegetable oils into solid fats, a process known as partial hydrogenation, some unsaturated bonds are converted to an unnatural trans position. In humans, trans fatty acids increase low-density lipoprotein cholesterol and decrease high-density lipoprotein cholesterol. In addition, positive associations between intake of trans fatty acids and coronary heart disease have been observed in epidemiological studies. The combined results of metabolic and epidemiological studies provide strong evidence that trans fatty acid intake is causally related to risk of coronary disease. Because the consumption of partially hydrogenated fats is almost universal in the United States, the number of deaths attributable to such fats is likely to be substantial. Federal regulations should require manufacturers to include trans fatty acid content in food labels and should aim to greatly reduce or eliminate the use of partially hydrogenated vegetable fats.