Background: Cognitive-behavioural therapy (CBT)-based guided self-help is recommended as a first step in the treatment of bulimia nervosa. Aims: To evaluate in a randomised controlled trial (Clinicaltrials.gov registration number: NCT00461071) the long-term effectiveness of internet-based guided self-help (INT-GSH) compared with conventional guided bibliotherapy (BIB-GSH) in females with bulimia nervosa. Method: A total of 155 participants were randomly assigned to INT-GSH or BIB-GSH for 7 months. Outcomes were assessed at baseline, month 4, month 7 and month 18. Results: The greatest improvement was reported after 4 months with a continued reduction in eating disorder symptomatology reported at month 7 and 18. After 18 months, 14.6% (n = 7/48) of the participants in the INT-GSH group and 25% (n = 7/28) in the BIB-GSH group were abstinent from binge eating and compensatory measures, 43.8% (n = 21/48) and 39.2% (n = 11/28) respectively were in remission. No differences regarding outcome between the two groups were found. Conclusions: Internet-based guided self-help for bulimia nervosa was not superior compared with bibliotherapy, the gold standard of self-help. Improvements remain stable in the long term. © The British Journal of Psychiatry 2013.