Various authors (e.g. Kahnemann, D., & Tversky, A. (1982). The simulation heuristic. In D. Kahnemann, P. Slovic, & A. Tversky (Eds.), Judgment under uncertainty: Heuristics and biases (pp. 201-208). New York, NY: Cambridge University Press) have suggested that imagining an event and its consequences influences the perceived likelihood that it might happen in reality (simulation heuristic). A scenario--a description of how a certain activity can lead to a certain outcome--may stimulate one to imagine the outcomes and may influence one's likelihood judgement. The present research studied the effect of risk scenarios on perceptions of susceptibility to sexually transmitted infections and the role of imaginability therein. In a randomised experimental study, we examined the effects of a prefabricated risk scenario and a self-constructed risk scenario against a non-message condition on perceived susceptibility to get infected with Chlamydia. Participants considered themselves more susceptible to Chlamydia after writing their own risk scenario but not after reading the prefabricated risk scenario. The imaginability of the event seemed to mediate the effect of self-constructed scenario information on perceived susceptibility. Recommendations for health education practices are discussed.