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Background. Cognitive behaviour therapy (CBT) is an effective treatment for obsessive--compulsive disorder (OCD) but access to CBT is limited. Internet-based CBT (ICBT) with therapist support is potentially a more accessible treatment. There are no randomized controlled trials testing ICBT for OCD. The aim of this study was to investigate the efficacy of ICBT for OCD in a randomized controlled trial. Method. Participants (n = 101) diagnosed with OCD were randomized to either 10 weeks of ICBT or to an attention control condition, consisting of online supportive therapy. The primary outcome measure was the Yale-Brown Obsessive Compulsive Scale (YBOCS) administered by blinded assessors. Results. Both treatments lead to significant improvements in OCD symptoms, but ICBT resulted in larger improvements than the control condition on the YBOCS, with a significant between-group effect size (Cohen's d) of 1.12 (95 % CI 0.69-1.53) at post-treatment. The proportion of participants showing clinically significant improvement was 60% (95% CI 46-72) in the ICBT group compared to 6% (95% CI 1-17) in the control condition. The results were sustained at follow-up. Conclusions. ICBT is an efficacious treatment for OCD that could substantially increase access to CBT for OCD patients. Replication studies are warranted. [ABSTRACT FROM AUTHOR]