BACKGROUND:There is well-established evidence that Mental Health First Aid (MHFA) training improves knowledge about how to support someone developing a mental health problem, but less evidence that this support improves the mental health of the recipient of aid. This randomised controlled trial aimed to assess the long-term effects of MHFA training of parents on the mental health of their adolescent children. METHODS:384 Australian parents of an adolescent aged 12-15 were randomised to receive either the 14-h Youth MHFA course or the 15-h Australian Red Cross Provide First Aid course. Outcomes were assessed at baseline, 1-year, and 2-year follow-up in both parents and adolescents. Primary outcomes were cases of adolescent mental health problems, and parental support towards their adolescent if they developed a mental health problem, rated by the parent and adolescent. Secondary outcomes included parent knowledge about mental health problems, intentions and confidence in supporting a young person, stigmatizing attitudes, and help-seeking for mental health problems. RESULTS:Parent and adolescent reports showed no significant difference between training groups in the proportion of cases of adolescents with a mental health problem over time (ps > .05). There was also no significant difference between training groups in the quality of parental support provided to their adolescent at 1- or 2-year follow-up (ps > .05). In contrast, some secondary outcomes showed benefits from the Youth MHFA training relative to the control, with increased parental knowledge about mental health problems at 1-year (d = 0.43) and 2-year follow-up (d = 0.26), and increased confidence to help a young person (d = 0.26) and intentions to provide effective support (d = 0.22) at 1-year follow-up. CONCLUSIONS:The study showed some improvements in mental health literacy in training recipients, but could not detect changes in the mental health of adolescents and the support provided to them by their parents if they had a mental health problem. However, there was a lack of power to detect primary outcome effects and therefore the question of whether MHFA training leads to better outcomes in the recipients of aid remains to be further explored. TRIAL REGISTRATION:ACTRN12612000390886 , registered retrospectively 5/4/2012.