Objective Breakfast consumption has been associated with better mental health in adulthood, but the relationship between breakfast and mental health in adolescence is less well known. The aims of the present study were to evaluate breakfast quality in a cohort of adolescents and to investigate associations with mental health.Design Cross-sectional population-based study. Breakfast quality was assessed by intake of core food groups at breakfast, as determined from 3 d food diaries. Mental health was assessed using the Child Behaviour Checklist (CBCL), with higher scores representing poorer behaviour.Setting The Western Australian Pregnancy Cohort (Raine) Study, Perth, Western Australia.Subjects Eight hundred and thirty-six males and females aged between 13 and 15 years.Results Mean mental health score as assessed by the CBCL was 45·24 (sd 11·29). A high-quality breakfast consisting of at least three food groups was consumed by 11 % of adolescents, while 7 % of adolescents did not consume any items from core food groups on average over the 3 d period. The two most common core food groups consumed at breakfast in this population were dairy products followed by breads and cereals. For every additional food group eaten at breakfast, the associated total mental health score decreased by 1·66 (95 % CI −2·74, −0·59) after adjustment for potential confounding factors, representing an improvement in mental health score.Conclusion These findings support the concept that breakfast quality is an important component in the complex interaction between lifestyle factors and mental health in early adolescence.