Effective communication of treatment risks is important to enable patients to make informed decisions. This study aimed to determine the effects of different risk formats on participants' evaluation and interpretation of risk information and on their treatment choice. Participants (N=44) were recruited among patients who had undergone surgery for an abdominal aneurysm and were asked to evaluate treatment risks (surgery or an observation policy) of two hypothetical cases presented in one of three risk formats (numbers, vertical bars or icons). Risk information presented in vertical bars was evaluated as the most difficult to comprehend, and the perceived threat of this information was evaluated as higher than that of the other risk formats. Risk information presented as icons was evaluated as more helpful for making a decision, but resulted in a lower percentage of participants choosing for surgery than when risks were presented in the other formats. In conclusion, this study showed that different risk formats have different effects on participants' evaluation of the information and on their choice. Doctors should therefore be careful in choosing the format in which they present treatment risks.