Within the AAIR Program of the EU titled Evaluation of the Ingestion of Trans Fatty Acids (FA) and its association with cardiovascular risk factors in European countries (TRANSFAIR), which is being carried out in 16 countries with different alimentary habits, among which is our country, we have developed the following study. Based on the information derived from the last National Nutrition and Feeding Study (ENNA-3), we have made up a list of foods which make up 95% of the total ingestion of lipids, and it also includes those which although they are not included within this percentage, may have an especially high trans isomer content as a result of their processing. The foods selected for the analysis belong to different food groups: cereals, milk products, oils and fats, meats, various, and pre-cooked foods, until making up a total of 100 foods for each country. The central analysis laboratory is that of the Department of Human Nutrition, TNO Nutrition and Food Research, Zeist (The Netherlands). In each sample, in addition to the total lipids, one determines the saturated and unsaturated fatty acids, including the cis and trans isomers. The trans FA's measured were: C14:1 T9, C16:1 T9, C18:1 T, C18:2 T, C18:3 T + C20:1 T, C20:2 T11,14, and C22:1 T13. Of the samples analyzed, the highest percentages of trans FA with respect to the total FA corresponded to the following foods: French fries, pre-cooked and frozen croquettes sliced bread, margarine, cakes, and frozen mille feuilles dough of different industrial brands. The lowest percentages of trans FA's were found in refined vegetable oils (sunflower and olive), those used for deep frying, and those discarded in catering, as well as in some commercial brands of cookies and ice creams. Pure chocolate, different brands of sweetened powdered cocoa, and ready to make chocolate, did not contain and trans FA.