Introduction: Over the last decades, many Western countries have seen an increase in mental health difficulties in the adolescent population. Among Norwegian adolescents, internalizing symptoms are most prevalent, and 10%-38% of adolescents seek out primary health care services for mental health problems. This study examined whether an idiographic measure designed for adolescents called Assert, which uses variables individually selected by the adolescents, could improve user involvement, locus of control, mental health, and quality of life. Methods: This study conducted a randomized controlled effectiveness trial using Assert in primary healthcare. A community sample of 150 Norwegian adolescents aged 12-23 years (M= 16.2; SD = 2.2) were randomized to the Assert or control group. Of the participants 27 (18%) were male. The adolescents and 52 counsellors from primary healthcare services responded to online questionnaires. Of the counselors, 92.8% were female and trained in the use of systematic idiographic assessment. Results: Adolescents who used Assert scored significantly lower on external LoC, attributing less of their mental health improvement to chance, fate, or luck. However, the intervention showed no effect on the symptoms of mental health, QoL, or user involvement. Conclusions: Using Assert in primary healthcare may enhance adolescents' sense of being able to influence their life circumstances and mental health by allowing them more control of their treatment. However, this approach might not be successful in reducing symptoms."What Matters to You?": A Randomized Controlled Effectiveness Trial, Using Systematic Idiographic Assessment as an Intervention to Increase Adolescents' Perceived Control of their Mental Health.Of all mental illnesses diagnosed in adults, 75% has its onset in adolescence (Ruiz & Primm, 2010). In the last few decades, many Western countries have seen a trend of increasing mental health difficulties in adolescents (Kim & Hagquist, 2018) and a higher prevalence of emotional disorders, especially among young women (Collishaw, 2015). These trends also manifest in Norway, where about 6% of boys and 19.7% of girls experience significant self-reported mental health problems; these figures have increased by about 4% since 2011 (Bakken, 2017). According to a Norwegian national survey, 15.2% of high school-age adolescents were troubled by symptoms of depression (Bakken, 2019). Among high school students, 34%-38% visited a school health nurse in 2017, and 10%-20% visited a low threshold adolescent health clinic (