BackgroundAnxiety disorders affect approximately 10% to 20% of young people, can be enduring if left untreated, and have been associated with psychopathology in later life. Despite this, there is a paucity of empirical research to assist clinicians in determining appropriate treatment options. We describe a protocol for a randomized controlled trial in which we will examine the effectiveness of a group-based Acceptance and Commitment Therapy program for children and adolescents with a primary diagnosis of anxiety disorder. For the adolescent participants we will also evaluate the elements of the intervention that act as mechanisms for change.Methods/designWe will recruit 150 young people (90 children and 60 adolescents) diagnosed with an anxiety disorder and their parent or caregiver. After completion of baseline assessment, participants will be randomized to one of three conditions (Acceptance and Commitment Therapy, Cognitive Behavior Therapy or waitlist control). Those in the Acceptance and Commitment Therapy and Cognitive Behavior Therapy groups will receive 10 × 1.5 hour weekly group-therapy sessions using a manualized treatment program, in accordance with the relevant therapy, to be delivered by psychologists. Controls will receive the Cognitive Behavior Therapy program after 10 weeks waitlisted. Repeated measures will be taken immediately post-therapy and at three months after therapy cessation.DiscussionTo the best of our knowledge, this study will be the largest trial of Acceptance and Commitment Therapy in the treatment of children and young people to date. It will provide comprehensive data on the use of Acceptance and Commitment Therapy for anxiety disorders and will offer evidence for mechanisms involved in the process of change. Furthermore, additional data will be obtained for the use of Cognitive Behavior Therapy in this population and this research will illustrate the comparative effectiveness of these two interventions, which are currently implemented widely in contemporary clinical practice. Anticipated difficulties for the trial are the recruitment and retention of participants, particularly adolescents. To avert these concerns and maximize recruitment, several strategies will be adopted to optimize referral rates as well as reduce participant drop-outs.Trial registrationThis trial is registered with the Australian and New Zealand Clinical Trials Registry, registration number: ACTRN12611001280998