Background: Cognitive behaviour therapy (CBT) has been shown to be an effective treatment for anxiety disorders in children, but few affected seek or receive treatment. Internet-delivered CBT (ICBT) could be a way to increase the availability of empirically supported treatments. Aims: A randomised controlled trial was conducted to evaluate ICBT for children with anxiety disorders. Method: Families (N = 93) with a child aged 8-12 years with a principal diagnosis of generalised anxiety disorder, panic disorder, separation anxiety, social phobia or specific phobia were recruited through media advertisement Participants were randomised to 10 weeks of ICBT with therapist support, or to a waitlist control condition. The primary outcome measure was the Clinician Severity Rating (CSR) and secondary measures included child-and parent-reported anxiety. Assessments were made at pre-treatment, post-treatment and at three-month follow-up. Results: At post-treatment, there were significant reductions on CSR in the treatment group, with a large between-group effect size (Cohens d = 1.66). Twenty per cent of children in the treatment group no longer met criteria for their principal diagnosis at post-treatment and at follow-up this number had increased to 50%. Parent-reported child anxiety was significantly lower in the treatment group than in the waitlist group at post-treatment, with a small between-group effect size (Cohens d = 0.45). There were no significant differences between the groups regarding child-ratings of anxiety at post-treatment. Improvements were maintained at three-month follow-up, although this should be interpreted cautiously due to missing data. Conclusions: Within the limitations of this study, results suggest that ICBT with therapist support for children with anxiety disorders can reduce clinician-and parent-rated anxiety symptoms. Trial registration: Clinicaltrials.gov: NCT01533402. (C) 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licensesiby-nc-nd/4.0/).