Background: Major depression can be treated by means of cognitive-behavioural therapy, but as skilled therapists are in short supply there is a need for self-help approaches. Many individuals with depression use the internet for discussion of symptoms and to share their experience. Aims: To investigate the effects of an internet-administered self-help programme including participation in a monitored, web-based discussion group, compared with participation in web-based discussion group only. Method: A randomised controlled trial was conducted to compare the effects of internet-based cognitive-behavioural therapy with minimal therapist contact (plus participation in a discussion group) with the effects of participation in a discussion group only. Results: Internet-based therapy with minimal therapist contact, combined with activity in a discussion group, resulted in greater reductions of depressive symptoms compared with activity in a discussion group only (waiting-list control group). At 6 months' follow-up, improvement was maintained to a large extent. Conclusions: Internet-delivered cognitive-behavioural therapy should be pursued further as a complement or treatment alternative for mild-to-moderate depression.