Objective: To (1) estimate the lifetime and 12-month prevalence of self-harm without suicide intent in young people aged 12Ð17 years, (2) describe the co-morbidity of these behaviours with mental illness and (3) describe their co-variation with key social and demographic variables. Method: A nationally representative random sample of households with children aged 4Ð17 years recruited in 2013Ð2014. The survey response rate was 55% with 6310 parents and carers of eligible households participating. In addition, 2967 (89%) of young people aged 11Ð17 completed a self-report questionnaire with 2653 of the 12- to 17-year-olds completing questions about self-harm behaviour. Results: In any 12-month period, about 8% of all 12- to 17-year-olds (an estimated 137,000 12- to 17-year-olds) report engaging in self-harming behaviour without suicide intent. This prevalence increases with age to 11.6% in 16- to 17-year-olds. Eighteen percent (18.8%; 95% confidence interval CI = 14.5, 23.0) of all 12- to 17-year-old young people with any mental health disorder measured by parent or carer report said that they had engaged in self-harm in the past 12 months. Among young people who were measured by self-report and met criteria for the Diagnostic and Statistical Manual of Mental DisordersÕ major depressive disorder almost half (46.6%; 95% CI = 40.0, 53.1) also reported that they had engaged in self-harm in the past 12 months. Suicide risk among those who self-harm is significantly elevated relative to the general population. Conclusion: The demonstrated higher risks in these young people for continued harm or possible death support the need for ongoing initiatives to reduce self-harm through mental health promotion, improved mental health literacy and continuing mental health reform to ensure services are accessible to, and meet the needs of families and young persons.