Objectives Treatment effects on post-traumatic growth (PTG) and its subdomains were investigated together with predictors of the Janus-face model of PTG. Design Effects were investigated within a randomized controlled trial of cognitive-behavioural theropy (CBT) for post-traumatic stress disorder (PTSD). Method Forty motor vehicle accident survivors were randomly assigned to a treatment or waiting condition. PTG was measured by the Post-traumatic Growth Inventory and complemented by its possible predictors (optimism, openness). Results The CBT treatment proved to be highly effective in terms of PTSD symptom reduction. In contrast to previous findings, however, there was no treatment effect on PTG in general. The CBT group showed, however, increases in PTG subdomains 'new possiblities' and 'personal strength'. Conclusions The results of this study caution researchers to naively expect PTG as a uniformly positive outcome to evaluate treatment effectiveness.