Several physical, psychological and behavioural changes may affect food habits during adolescence and have long-term consequences on adult health status. Also, as food habits are related to lifestyle and physical activity, all should be assessed together. This paper describes a self-administered food frequency questionnaire (FFQ) designed to assess semi-quantitatively food habits of adolescents, and evaluates its use in a study of lifestyle and physical activity. A FFQ was developed, tested in 20 adolescents and compared with a modified version of the diet history method (a combination of a 3-day dietary record and an interview with a dietitian). This validated semi-quantitative questionnaire was later included in a larger questionnaire on lifestyle and physical activity in a study of 3540 adolescents aged 9-19 years. In the validation study with 20 adolescents, the FFQ showed a good agreement with the modified version of the diet history. During the survey several consumption frequencies were found to be low. In the group of adolescents aged 14-19 years old, dairy products were consumed daily by less than 50% of the sample. About 53% girls but only 33% boys consumed one fruit daily. For one vegetable portion, these proportions were 17 and 8%, respectively. The self-administered food frequency questionnaire correctly describes food consumption in adolescents. Moreover, it was well accepted by the target group, easily understood and completed with very few problems. The results show that a significant proportion of adolescents didn>>t consume milk, fruit and vegetable on a daily basis.