Background: Posttraumatic stress disorder (PTSD) is a severe and disabling condition and few receive appropriate care. Internet-based treatment of PTSD shows promise in reducing barriers to care and preliminary evidence suggests it is efficacious in treating symptoms of PTSD. Methodology: Forty-two individuals with a diagnosis of PTSD confirmed by clinician interview completed a randomized controlled comparison of Internet-based cognitive behavioral therapy (CBT) with a waitlist control condition. Principle Findings: Large pre- to posttreatment effect sizes (ESs) were found for the Treatment group on measures of PTSD symptoms, depression, anxiety, and disability. A small between-group ES was found for PTSD symptoms and moderate between-group ESs were found for depression, anxiety, and disability. Conclusions: Results provide preliminary support for Internet-based CBT as an efficacious treatment for individuals with a confirmed primary diagnosis of PTSD. Depression and Anxiety, 2011.