Advances in communication technology offer additional strategies for providing psychological treatment. Previous trials of Internet-based treatment approaches reported significant reductions in posttraumatic stress and related symptoms in response to Internet-based treatments relative to control groups. However, empirical data on the long-term effects of those approaches are sparse. In order to evaluate the long-term effect of an Internet-based intervention, the authors conducted an 18-month follow-up of an Internet-based cognitive behavioural therapy for posttraumatic stress. Severity of posttraumatic stress symptoms was the primary outcome. Additional measures were depression, anxiety, mental and physical health, and health care utilization during the follow-up period. Treatment group participants (n = 34) were assessed 1.5 years after completing treatment. Results indicated that reductions in symptoms of posttraumatic stress symptoms, depression, and anxiety found at posttreatment were sustained during the 18-month follow-up period. Preliminary evidence on long-term effects of Internet-based health care as shown in this study is promising. However, research with larger and clinically more diverse samples is needed to fully assess the clinical impact and potential of Internet-based health care provision.