ABSTRACT: OBJECTIVES:Social anxiety disorder (SAD) is one of the most common psychiatric disorders in youth, with a prevalence of about 3%-4% and increased risk of adverse long-term outcomes, such as depression. Cognitive-behavioural therapy (CBT) is considered the first-line treatment for youth with SAD, but many adolescents remain untreated due to limited accessibility to CBT. The aim of this study was to develop and evaluate the feasibility and preliminary efficacy of a therapist-guided internet-delivered CBT treatment, supplemented with clinic-based group exposure sessions (BIP SOFT). DESIGN:A proof-of-concept, open clinical trial with 6-month follow-up. PARTICIPANTS:The trial was conducted at a child and adolescent psychiatric research clinic, and participants (n=30) were 13-17 years old (83% girls) with a principal diagnosis of SAD. INTERVENTION:12 weeks of intervention, consisting of nine remote therapist-guided internet-delivered CBT sessions and three group exposure sessions at the clinic for the adolescents and five internet-delivered sessions for the parents. RESULTS:Adolescents were generally satisfied with the treatment, and the completion rate of internet modules, as well as attendance at group sessions, was high. Post-treatment assessment showed a significant decrease in clinician-rated, adolescent-rated and parent-rated social anxiety (d=1.17, 0.85 and 0.79, respectively), as well as in general self-rated and parent-rated anxiety and depression (d=0.76?and 0.51), compared with pretreatment levels. Furthermore, 47% of participants no longer met Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) criteria for SAD at post-treatment. At a 6-month follow-up, symptom reductions were maintained, or further improved, and 57% of participants no longer met criteria for SAD. CONCLUSION:Therapist-guided and parent-guided internet-delivered CBT, supplemented with a limited number of group exposure sessions, is a feasible and promising intervention for adolescents with SAD. TRIAL REGISTRATION NUMBER:NCT02576171; Results.