Existing studies suggest that mindfulness-based interventions (MBIs) can reduce ADHD symptoms when delivered as targeted clinical programs to young people diagnosed with ADHD. However, there is currently a lack of research on whether MBIs are effective in reducing elevated ADHD symptoms when delivered as universal programs to whole classes in schools. This study investigated the effects of a universal nine-week MBI (.b program) on students' elevated ADHD symptoms using an cluster randomized controlled trial (RCT) design. The modifying effects of pupils' baseline characteristics (gender, age, and socioeconomic status) and home-practice intensity were examined. A total of 3519 adolescents aged 12 to 15 years from Finnish schools were randomized into an MBI group, a relaxation-based active-control group, and a passive-control group. ADHD symptoms were measured at baseline (T0), at nine weeks (T9), and at 26-weeks (T26, follow-up) using parent-reported assessments. The study found no positive intervention effects on parent-reported ADHD symptoms in the MBI group compared to the control groups. Neither boys nor girls benefited from the MBI. The same was true of different age groups and socioeconomic levels. Moreover, regular home practice during the follow-up period did not strengthen the intervention effects. The results do not support the use of brief, universal MBIs in whole school classes as part of the routine curriculum to reduce elevated ADHD symptoms in adolescents. Further research is essential to identify optimal content and delivery methods for MBIs in schools. Particularly among adolescents with elevated ADHD symptoms, targeted MBIs, adapted to the unique needs of this subgroup, should be examined.

Trial Registration: Healthy Learning Mind-a school-based mindfulness and relaxation program: a study protocol for a cluster randomized controlled trial (RCT) ISRCTN18642659 retrospectively registered on 13 October 2015. The full trial protocol can be accessed at