Political interest in prevention of mental illness has increased in recent years. However, relatively little is known about the public s beliefs about prevention, and the predictors of these beliefs. Since many disorders start in the first decades of life, a focus on young people is warranted. Methods: Young people s prevention beliefs were assessed by a national telephone survey of 3746 Australian youths aged 12-25 years in 2006. A similar survey was repeated in 2011 with 3021 youths aged 15-25. In both surveys, respondents were presented with a vignette portraying depression, psychosis, social phobia, or depression with alcohol abuse in a young person. The 2011 survey also included depression with suicidal thoughts and post-traumatic stress disorder. Respondents rated the helpfulness of seven potential prevention strategies, and reported on any experience of mental health problems and treatment in the past year, exposure to beyondblue and mental health information at school or work. Results: Most respondents believed that regular contact with friends and family and regular physical activity would be helpful. Respondents who had recently experienced mental health problems, younger respondents, females, and those not exposed to beyondblue or mental health information were more likely to hold beliefs that differed from those of health professionals or available evidence. No significant changes were observed between surveys. Limitations: Actual preventive actions and reasons behind respondents beliefs were not assessed. Conclusions: Future prevention efforts should target subgroups with beliefs that differ from professionals and research evidence. Beyondblue and school and work settings may be promising avenues for these efforts.