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. Copyright © 2012 John Wiley & Sons, Ltd. Key Practitioner Message In comparison with no treatment and unsupported bibliotherapy, online CBT induces strong reductions in bulimic symptoms.Internet‐delivered treatment may provide an acceptable treatment alternative for bulimic patients who are reticent about face‐to‐face contact.Therapist support appears to be a critical determinant of treatment adherence and effectiveness.Unsupported bibliotherapy may have only small immediate effects but may increase the probability of recovery in the long term by promoting positive attitudes towards treatment.