Objective: The aim of the present study was to determine the efficacy of an Internet-based clinician-assisted computerized cognitive behavioural treatment (CaCCBT) programme for depression. Method: Forty-five individuals meeting diagnostic criteria for depression were randomly assigned to the Sadness programme or to a waitlist control group. In the clinician-assisted Sadness programme, participants complete six online lessons, weekly homework assignments, receive weekly email contact from a clinical psychologist, and contribute to a moderated online discussion forum with other participants. An intention-to-treat model was used for data analyses. Results: A total of 20 (74%) treatment group participants completed all lessons within the 8 week programme, and post-treatment data were collected from 18/27 treatment group and 17/18 waitlist group participants. Treatment group participants reported significantly reduced symptoms of depression as measured by the Beck Depression Inventory–second edition and the Patient Health Questionnaire–Nine Item. Treatment group participants each received an average of eight email contacts (111 min of therapist time]. Mean within- and between-group effect sizes (Cohen's d) across the two measures of depressive symptoms were 0.98 and 0.75, respectively. Participants found the treatment programme acceptable and satisfactory. Conclusions: These results replicate those from the pilot trial reported by Perini et al. and are consistent with literature indicating that Internet-based programmes for depression and other mental disorders combined with clinical guidance can result in clinically significant improvements. These data provide further support for the development of Internet-based treatment for common mental disorders.