Abstract Background Members of the public know about actions they can take to prevent major physical diseases, but there has been less attention to the public's capacity to take action to prevent mental disorders. Since mental disorders often have first onset during youth, young people's beliefs about prevention are of particular relevance. Methods Young people's prevention beliefs were assessed by a national telephone survey of 3746 Australian youth aged 12–25 years. To allow a comparison with professional beliefs, postal surveys were carried out with 470 GPs, 591 psychiatrists, 736 psychologists and 522 mental health nurses. Respondents were asked to rate the helpfulness of 9 potential strategies in relation to the prevention of four disorders: depression, depression with alcohol misuse, social phobia and psychosis. Results Both young people and professionals believed that mental disorders could be prevented by physical activity, keeping contact with family and friends, avoiding use of substances, and making time for relaxing activities. However, professionals disagreed with young people about the benefits of avoiding stressful situations, particularly for social phobia. Professionals were also less optimistic about the prevention of psychosis than depression and anxiety. Limitations The surveys assessed beliefs, but not actual use of preventive strategies. Conclusions Given the beliefs of young people and professionals that prevention is possible, there is fertile ground for health promotion in this area. However, young people need to be aware that avoiding stressful situations may not be helpful for anxiety.