In a previous paper we predicted that health effects of dietary fats in humans would require half a century or more to be understood, instead of the decade or so predicted during 1956 by an Editorial in The Lancet. It would seem that our prediction may have been optimistic since it has now been reported that trans unsaturated fatty acids present in high concentrations in margarines promote hypercholesterolemia in humans. Consequently, there has been a call for the reclassification of dietary fats upon the basis of their hypercholesterolemic properties. Using the latter criterion, therefore, many margarine brands would be classified as coronary artery disease risk foods. The primary adverse metabolic action of trans unsaturated fatty acids is the competitive inhibition of delta-6-desaturase, the hepatic enzyme responsible for the initial metabolic desaturation of the essential fatty acids cis linoleic and cis alpha-linolenic acid. In addition to margarines, many other common foods such as deep-fried foods, many convenience foods and bakery products contain relatively high levels of trans fatty acids. Therefore, since it has become virtually impossible to avoid a consistent, daily dietary intake of trans fatty acids, it would appear that a precautionary, preventative supplementation of the diet with supplements containing the direct metabolic products of delta-6-desaturation of the essential fatty acids, would be prudent. Such supplements are readily available.