Objective: The purpose of the present study was to examine the effectiveness of an Internet-based clinician-assisted computerized cognitive behavioural therapy programme for social phobia. Method: A total of 105 individuals with social phobia were randomly assigned to a six-lesson cognitive behavioural treatment programme or to a waitlist control group. Treatment consisted of four components: six online lessons; homework assignments; participation in an online discussion forum; and regular email contact with a therapist. An intention-to-treat model was used for data analyses. Results: A total of 78% of treatment group participants completed all lessons, and post-treatment data were obtained from 93/105 participants. Significant post-treatment differences between treatment and waitlist participants were found on two measures of symptoms of social phobia. Mean within- and between-group effect sizes (Cohen's d) for the primary social phobia outcome measures were 1.15, and 0.95, respectively. Conclusions: These results were comparable with those obtained in exemplary face-to-face treatment programmes. They provide further positive data about the utility of Internet-based guided self-help programmes for people with common mental disorders.