BACKGROUNG/OBJECTIVES: A higher educational level is associated with a healthier diet. The goal of this study was to establish whether this association is mediated by attitudes toward healthy eating. SUBJECTS/METHODS: The cross-sectional MONA LISA-NUT study was performed in 2005-2007 on adults aged 35-64 years from northern and north-eastern France. Diet quality was assessed on the basis of a 3-day food record and a validated score based on French national dietary guidelines. Specific questions investigated attitudes toward healthy eating. Multivariate analyses were used to quantify the proportion of the educational level-diet relationship that was mediated by attitudes toward healthy eating. RESULTS: Among the 1631 subjects, favourable attitudes toward healthy eating were associated with both higher educational level and diet quality. In the mediation analysis, 'organic food consumption' explained 14% (95% confidence interval (8;24)) of the educational level-diet relationship and 'attention paid to health when buying food' explained 9% (3;16). In contrast, 'attention to food choice', 'searching for information about food' and 'perceived role of eating' were not mediators of the association between educational level and diet. In a multivariate model, the attitude items together accounted for 25% (10;45) of the relationship. The mediation was more pronounced in women than in men (37% (15;54) vs 16% (1;27), respectively) and was significant for consumption of fruits and vegetables (23% (13;37)), whole-grain food (32% (15;58)) and seafood (22% (9;55)). CONCLUSIONS: Our results suggest that poor attitudes toward healthy eating in groups with low socioeconomic status constitute an additional factor (along with cost constraints) in the choice of unhealthy foods.