The present studies test the hypothesis that the degree of experienced ambivalence toward health behaviors moderates the impact of differently framed messages. In line with prospect theory, it is argued that positive frames can either involve attaining desirable outcomes or avoiding undesirable outcomes, and negative frames can either emphasize the presence of undesirable outcomes or the absence of desirable outcomes. The results of three studies are supportive of the hypothesis that highly ambivalent individuals are more persuaded by negatively framed messages whereas individuals low in ambivalence are more persuaded by positively framed messages. The greater persuasiveness of negatively framed messages at higher levels of ambivalence can be explained by a negativity bias involved in ambivalence. Several preventive behaviors such as eating a low-fat diet or using condoms were addressed. Theoretical and practical implications of these findings are discussed in light of current theories on health behavior. Copyright © 2002 John Wiley & Sons, Ltd.