Objective: To determine the efficacy of an Internet-based clinician-assisted cognitive behavioural treatment program (the Panic program) for panic disorder (with or without agoraphobia). Method: Fifty-nine individuals meeting diagnostic criteria for panic disorder with agoraphobia were randomly assigned to a treatment group or to a waitlist control group. Treatment group participants completed the Panic program, comprising six on-line lessons, weekly homework assignments, received weekly email contact from a psychiatry registrar, and contributed to a moderated online discussion forum with other participants. An intention-to-treat model was used for data analyses. Results: Twenty-three (79%) of treatment group participants completed all lessons within the 8-week program, and post-treatment data were collected from 22/29 treatment group and 22/25 waitlist group participants. Compared to the control group, treatment group participants reported significantly reduced symptoms of panic as measured by the Panic Disorder Severity Scale, Body Sensation Questionnaire, and Agoraphobic Cognitions Questionnaires. Significant reductions were also reported on measures of disability and depression. The mean within- and between-group effect size (Cohen's d) on the Panic Disorder Severity Scale was 0.93 and 0.59, respectively, and effects were sustained at 1-month follow-up. Mean therapist time per participant was 75 minutes for the program. Conclusions: These results replicate those from the open trial of the Panic Program indicating the efficacy of the Internet-based clinician-assisted cognitive behavioural treatment program for panic disorder with agoraphobia.