The aim of this study was to examine the effect of an Internet-based CBT intervention on posttraumatic growth, openness and optimism. Ninety-six participants suffering from posttraumatic stress reactions were randomly assigned to either the treatment or a waiting-list control condition. We assessed posttraumatic stress reactions, depression and anxiety, posttraumatic growth, optimism and openness to new experiences. We found significant changes in posttraumatic growth in the treatment group. In addition regression analysis revealed that symptom improvements in posttraumatic stress reactions significantly predicted posttraumatic growth post-treatment. Our findings indicate the potential of psychotherapy to stimulate posttraumatic growth.