Objective: The purpose of the study was to evaluate an Internet-based cognitive behavioral therapy (iCBT) program targeting difficulties and impairments associated with adult ADHD. Method: Forty-five adults diagnosed with ADHD were randomized to either self-help (iCBT self-help format [iCBT-S]), self-help with weekly group sessions (iCBT group-therapy format [iCBT-G]), or a waiting-list control group. Treatment efficacy was measured at pre- and posttreatment and at 6-month follow-up. Results: Intention-to-treat (ITT) analysis showed a significant reduction in ADHD symptoms for the iCBT-S group in comparison with the waiting-list controls at posttreatment, with a between-group effect size of d = 1.07. The result was maintained at 6-month follow-up. No significant difference was found at posttreatment or 6-month follow-up between the iCBT-S and iCBT-G groups. Conclusion: The findings show that a CBT treatment program administered through the Internet can be a promising treatment for adult ADHD. Limitations of the study design and directions for future research are discussed.