The goal of this clinical trial is to compare therapist-guided internet delivered prolonged exposure to an active control condition( therapist-guided internet delivered cognitive behavioral therapy containing relaxation techniques) for post-traumatic stress disorder. The objective with this study is to investigate efficacy, mechanisms of change and cost effectiveness of therapist-guided internet delivered prolonged exposure. Adult patients with post-traumatic stress disorder will be randomly assigned to receive either 10 weeks of therapist-guided internet delivered prolonged exposure or therapist-guided internet delivered cognitive-behavioral therapy containing relaxation.