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.