Background: Depression in adolescents is a significant problem that impairs everyday functioning and increases the risk of severe mental health disorders in adulthood. Although this is a major problem, relatively few adolescents with, or at risk of developing, depression are identified and referred for treatment. This suggests the need to investigate alternative approaches whereby preventative interventions are made widely available in schools. Objective: To investigate the clinical effectiveness and cost-effectiveness of classroom-based cognitive- behavioural therapy (CBT) in reducing symptoms of depression in high-risk adolescents. Design: Cluster randomised controlled trial. Year groups (n = 28) randomly allocated on a 1: 1: 1 basis to one of three trial arms once all schools were recruited and balanced for number of classes, number of students, Personal, Social and Health Education (PSHE) lesson frequency, and scheduling of PSHE. Setting: Year groups 8 to 11 (ages 12-16 years) in mixed-sex secondary schools in the UK. Data were collected between 2009 and 2011. Participants: Young people who attended PSHE at participating schools were eligible (n = 5503). Of the 5030 who agreed to participate, 1064 (21.2%) were classified as 'high risk': 392 in the classroom-based CBT arm, 374 in the attention control PSHE arm and 298 in the usual PSHE arm. Primary outcome data on the high-risk group at 12 months were available for classroom-based CBT (n = 296), attention control PSHE (n = 308) and usual PSHE (n = 242). Interventions: The Resourceful Adolescent Programme (RAP) is a focused CBT-based intervention adapted for the UK (RAP-UK) and delivered by two facilitators external to the school. Control groups were usual PSHE (usual school curriculum delivered by teachers) and attention control (usual school PSHE with additional support from two facilitators). Interventions were delivered universally to whole classes. Primary outcomes: Clinical effectiveness: symptoms of depression [Short Mood and Feelings Questionnaire (SMFQ)] in adolescents at high risk of depression 12 months from baseline. Cost-effectiveness: incremental cost-effectiveness ratios (ICERs) based on SMFQ score and quality-adjusted life-years (from European Quality of Life-5 Dimensions scores) between baseline and 12 months. Process evaluation: reach, attrition and qualitative feedback from service recipients and providers. Results: SMFQ scores had decreased for high-risk adolescents in all trial arms at 12 months, but there was no difference between arms [classroom-based CBT vs. usual PSHE adjusted difference in means 0.97, 95% confidence interval (CI) -0.34 to 2.28; classroom-based CBT vs. attention control PSHE -0.63, 95% CI -1.99 to 0.73]. Costs of interventions per child were estimated at £41.96 for classroom-based CBT and £34.45 for attention control PSHE. Fieller's method was used to obtain a parametric estimate of the 95% CI for the ICERs and construct the cost-effectiveness acceptability curve, confirming that classroom-based CBT was not cost-effective relative to the controls. Reach of classroom-based CBT was good and attrition was low (median 80% attending ≥ 60% of sessions), but feedback indicated some difficulties with acceptability and sustainability. Conclusions: Classroom-based CBT, attention control PSHE and usual PSHE produced similar outcomes. Classroom-based CBT may result in increased self-awareness and reporting of depressive symptoms. Classroom-based CBT was not shown to be cost-effective. While schools are a convenient way of reaching a wide range of young people, implementing classroom-based CBT within schools is challenging, particularly with regard to fitting programmes into a busy timetable, the lack of value placed on PSHE, and difficulties engaging with teachers and young people. Wider use of classroom-based depression prevention programmes should not be undertaken without further research. If universal preventative approaches are to be pursued, their clinical effectiveness and cost-effectiveness with younger children (aged 10-11 years), before the incidence of depression increases, should be investigated. Alternatively, the clinical effectiveness of indicated school-based programmes targeting those already displaying symptoms of depression should be investigated.