Objective: To evaluate the effects of a universal prevention programme for disordered eating behaviours (DEB) and sedentary lifestyle among Mexican young people. Design: Non-experimental single group field study with repeated measures (pre-test, post-test and at 6-month follow-up). Setting: Public high school in Hidalgo, Mexico. Methods: In all, 214 young women and 154 young men aged 15--18 years with a mean age of 16.4 participated in the study. For both young women and men, data were collected using the Brief Questionnaire for Disordered Eating Behaviours (DEB) and the short form of the International Physical Activity Questionnaire. Among young men, additional data were collected using the Drive for Muscularity Scale (DMS). Intervention: Five activity-based sessions. In the first four sessions, the following topics were addressed: thinness culture, myths and realities of dieting and supplements, healthy eating behaviours and healthy menus. At the end of each session, students engaged in an enjoyable physical activity hour. In the fifth session, participants were divided into three parallel workshop groups focusing on: thinness culture, healthy menus, and physical activity. The programme incorporated a variety of dissonance-based interventions using interactive psychoeducational strategies. Results: After 6 months, repeated-measures analyses of variance (ANOVAs) revealed a significant reduction in the mean DEB scores in young women. In young men, DEB and DMS mean scores decreased but not significantly. Physical activity frequency and duration showed a significant increase in the total sample over time. Conclusions: Overall, implementation of the programme had positive effects on young people and important differences were found between the sexes.