Our objective was to study the trend in differences in the frequency of obesity by educational level in the general population 20 to 64 years of age. We used data from two cross-sectional health surveys carried out in 1987 and 1993 in representative samples of the Spanish population. We investigated the relation between obesity and educational level during the periods 1987 and 1993, taking into account the main factors confounding the relation. We used, as setting, the National Health Interview Surveys representative of the whole Spanish population. In both men and women, the highest odds ratios (ORs) for obesity were observed at lower educational levels. These differences increased in women between 1987 and 1993, while they decreased in men during the same period. Evidence of increased educational differences in the frequency of obesity indicates that future studies should focus on the evaluation and monitoring of this trend in the population.