Posttraumatic stress disorder (PTSD) is a debilitating mental health condition frequently associated with psychiatric comorbidity and diminished quality of life, and it typically follows a chronic, often lifelong, course. Previous research has shown that trauma-related psychopathology (but not necessarily clinical PTSD) can be effectively treated via the Internet. This study is the first of its kind to report on the online treatment of patients with a Diagnostic and Statistical Manual of Mental Disorders (fourth edition) clinical diagnosis of PTSD with therapist support by e-mail only. Preliminary findings are presented of an open trial involving a 10-week Internet-based therapist-assisted cognitive behavioural treatment for PTSD (PTSD Online). Pre and posttreatment measures of PTSD and related symptomatology were compared for 16 participants with a variety of trauma experiences. Participants showed clinically significant reductions in PTSD severity and symptomatology, moderate tolerance of the program content, and high therapeutic alliance ratings. No significant change was found on measures of more general psychological symptoms. The results suggest that PTSD Online appears to be an effective and accessible clinical treatment for people with a confirmed PTSD diagnosis.