The present study investigated the efficacy of self-help based on cognitive behaviour therapy in combination with Internet support in the treatment of bulimia nervosa and binge eating disorder. After confirming the diagnosis with an in-person interview, 73 patients were randomly allocated to treatment or a waiting list control group. Treated individuals showed marked improvement after 12 weeks of self-help compared to the control group on both primary and secondary outcome measures. Intent-to-treat analyses revealed that 37% (46% among completers) had no binge eating or purging at the end of the treatment and a considerable number of patients achieved clinically significant improvement on most of the other measures as well. The results were maintained at the 6-month follow-up, and provide evidence to support the continued use and development of self-help programmes.