Objective: To investigate the relationship between knowledge about a healthy diet and the actual food consumption habits of adolescents. Design: A survey of several food-related aspects applied to a representative sample of adolescents. Setting: One thousand, six hundred and sixty three individuals aged 11 to 18 from 40 schools randomly selected from among all the schools in the Balearic Islands. Method: The survey was administered under the supervision of a researcher during the school day. The adolescents were asked about proper eating habits and invited to classify 43 food items as healthy or not healthy. The subjects later reported the frequency with which they consumed these foods, as well as the types of products they consumed between meals. Results: The adolescents in the sample considered the abundant consumption of vegetables, fish and fruit to be the key to a healthy diet and, as of 15 years of age, recognized the need for a balanced, varied diet. They also correctly identified all the healthy foods, whereas typical fast food items, sweets and alcoholic drinks received highly negative ratings. However, only the girls' lower consumption of unhealthier products was consistent with this knowledge. The consumption of foods such as vegetables and fish by the older subjects even declined, despite their higher dietary ratings. Three-quarters of the subjects also reported that they customarily snack on calorie-rich food with scant nutritional value. Conclusion: While dietary ratings are consistent and seem to be reinforced by the constant messages in favour of the Mediterranean diet aimed at adults, the minimum impact of this knowledge on the subjects' habits observed in the study poses the need to seek new communicative strategies that are not exclusively based on providing adolescents with more information on proper diet. (Contains 5 tables.)