Background. Effective internet-based programs for depression usually incorporate a component that provides telephone or email contact. Open access websites, without such contact, show high rates of attrition and poorer outcomes. The present study was designed as an exploratory investigation of the parameters that influence the effectiveness and retention of users on open access websites. We investigated whether brief cognitive behaviour therapy (CBT) was as effective as an extended version, whether add-on components of behaviour therapy or stress management contributed to positive outcomes, and whether longer programs were associated with greater attrition.

Method. An online randomized controlled trial (RCT) was conducted between 13 January 2005 and 26 May 2005 (19 weeks). A total of 2794 registrants (1846 women and 948 men; median age category 35–44 years) with elevated scores on the Goldberg Depression Scale of 5·96 (S.D.=2·09) elected online to be randomized to one of six versions of a CBT website. The versions were compiled consisting of various components of brief CBT, extended CBT, behaviour strategies, stress management and problem solving.

Results. A total of 20·4% of participants completed the assigned intervention. The interaction of measurement occasion and treatment version was significant [F(13,131)=2·20, p=0·01]. A single module of brief introductory CBT was not effective in reducing depression symptoms. However, extended CBT with or without the addition of behaviour strategies resulted in the reduction of depression.

Conclusions. Brief CBT-based interventions are not as effective as extended interventions. However, longer programs are associated with higher rates of dropout.