Background: Internet-delivered treatment may reduce barriers to care in those unwilling or unable to access traditional forms of treatment. Objective: To assesses the efficacy of web-based therapist-assisted cognitive behavioral treatment (web-CBT) of panic symptoms. Design: A randomized waiting-list controlled trial with an uncontrolled three-year follow-up. Participants: A community sample of 58 participants with chronic panic symptoms of varying severity (immediate treatment: n=27, waiting-list control: n=31). Outcome measures: The primary outcome measures were a one-week Panic Diary and the Panic Disorder Severity Scale - Self-Report (PDSS-SR); secondary measures were the Agoraphobic Cognitions Questionnaire (ACQ), the Body Sensations Questionnaire (BSQ), the Mobility Inventory - Alone subscale (MI-AAL), and the Depression Anxiety Stress Scales (DASS-42). Results: In the RCT, 54 participants (93%) completed posttest measurements. With regard to the primary outcome measures, intention-to-treat ANCOVAs revealed that participants in the treatment condition improved more than the participants in the waiting-list control condition (p<03), with a pooled between-group effect size of d=7. After three years (n=47; 81% study compliance), effects were more pronounced. Conclusion: The results demonstrate the efficacy of therapist-assisted web-CBT in the treatment of panic symptoms.