The percentage of fat of a variety of foods served in fast food establishments has been determined. This percentage is very variable, with mean contents of total fat of 35.83 +/- 10.68% in beef hamburgers, 35.84 +/- 8.66% in chips, 23.02 +/- 5.07% in chicken hamburgers and 34.02 +/- 13.49% in "hot dogs". The lipidic composition is mainly formed by saturated fatty acids (28-52% of total fat) and monounsaturated (46-48%), whereas the proportion of polyunsaturated fatty acids showed more variable values, ranging from 6.9% in beef hamburgers to 25.1% in chips. Statistical treatment of the results of relative percentage of fatty acids by multivariate methods revealed clusters of samples grouped as a function of the type of food and in some instances of its source, which can be interesting in case of healthy problems. Percentages of trans fatty acids slightly higher than the maximum recommended values have been detected in some cases. These results show the urgent need of modifying the laws to force fast food establishments to specify the type and amounts of fat used in the processing of these foods, as well as a greater control from the local administrations.