Even though the efficacy of group-based cognitive behavioural interventions is well established both for adolescents diagnosed with major depressive disorders as well as for adolescents with depressive symptoms, in order to prevent further development, there has been a call for effectiveness studies in real world settings. This study investigated the effectiveness of a group-based cognitive behavioural early intervention implemented among 16-20 years old adolescents with subclinical or mild to moderate depression. Data were collected from spring 2010 to summer 2011, and were analysed by use of a benchmarking procedure. Calculations of effect sizes indicated medium to large reductions in symptoms of depression. The reduction was found to be maintained or even increased at follow-up six months later. The effect sizes were of the same magnitude as those from efficacy studies of group-based cognitive behavioural interventions for adolescent depression. Implications and limitations are discussed.