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Background Depression is a major contributor to the burden of disease in the adolescent population. Internet-based interventions can increase access to treatment.Aims To evaluate the efficacy of internet-based cognitive-behavioural therapy (iCBT), including therapist chat communication, in treatment of adolescent depression.Method Seventy adolescents, 15-19 years of age and presenting with depressive symptoms, were randomised to iCBT or attention control. The primary outcome was the Beck Depression Inventory II (BDI-II).Results Significant reductions in depressive symptoms were found, favouring iCBT over the control condition (F(1,67) = 6.18, P < 0.05). The between-group effect size was Cohen's d = 0.71 (95% CI 0.22-1.19). A significantly higher proportion of iCBT participants (42.4%) than controls (13.5%) showed a 50% decrease in BDI-II score post-treatment (P < 0.01). The improvement for the iCBT group was maintained at 6 months.Conclusions The intervention appears to effectively reduce symptoms of depression in adolescents and may be helpful in overcoming barriers to care among young people.Declaration of interest N.T. and G.A. designed the programme. N.T. authored the treatment material. The web platform used for treatment is owned by Linköping University and run on a non-for-profit basis. None of the authors receives any income from the programme. Copyright