Objective: The aim of this study was to identify differences in food habits and lifestyle behaviours by dieting status among young people in Ireland.Design: Cross-sectional survey. Participants responded to a self-completion questionnaire designed by researchers on the World Health Organization's collaborative study - Health Behaviour in School Aged Children. Pupils were selected by school and classroom and the sample was stratified to be representative of the geographical distribution of school students in Ireland.Setting: Data were collected by teachers from school pupils in their classrooms.Subjects: Data were collected from 187 schools which included 8497 pupils (51% girls) aged 9-17 years.Results: While a minority of pupils (12% of girls, 4% of boys) reported that they were on a diet to lose weight, a substantial proportion (28% of girls, 18% of boys) said that they should be on a diet. Dieters reported consuming unhealthy foods less frequently than non-dieters, but did not report an increased consumption of fruit and vegetables. Rather, some categories of dieters reported higher levels of coffee and tobacco use and lower exercise levels than non-dieters.Conclusions: The results could indicate substitution of unhealthy foods by other unhealthy behaviours as opposed to an increased consumption of healthier foodstuffs, and suggest that both smoking and exercise need to be addressed alongside nutrition in youth health promotion.