Background: Manualized cognitive-behavioural treatment (CBT) is underutilized in the treatment of bulimic symptoms. Internet-delivered treatment may reduce current barriers. Objective: This study aimed to assess the efficacy of a new online CBT of bulimic symptoms. Method: Participants with bulimic symptoms (n=105) were randomly allocated to online CBT, bibliotherapy or waiting list/delayed treatment condition. Data were gathered at pre-treatment, post-treatment and 1-year follow-up. Outcome Measures: The primary outcome measures were the Eating Disorder Examination Questionnaire (EDE-Q) and the frequency of binge eating and purging episodes. The secondary outcome measure was the Body Attitude Test. Results: Dropout from Internet treatment was 26%. Intention-to-treat ANCOVAs of post-test data revealed that the EDE-Q scores and the frequency of binging and purging reduced more in the online CBT group compared with the bibliotherapy and waiting list groups (pooled between-group effect size: d=0.9). At 1-year follow-up, improvements in the online CBT group had sustained. Conclusion: This study identifies online CBT as a viable alternative in the treatment of bulimic symptoms.