This study evaluates the impact on risk perception and trust in public health protection resulting from disclosure of information about implementation of precautionary measures and from the disclosure of scientific uncertainty in the area of mobile telephony. Based on an experimental design, the study supports our recent findings (Wiedemann and Schtz, 2005) that precautionary measures may trigger concerns and amplify EMF-related risk perceptions. Furthermore, our present data once again indicates that information about the implementation of precautionary measures has no positive effect on trust in public health protection. These results, contrary to common expectations, should be considered in decisions about precautionary measures. Risk managers who intend to implement precautionary measures merely as a means for reassuring the public will probably fail. Indeed, even if precautionary measures are justified from a public health perspective, it seems prudent to anticipate the possibly countervailing effects of such measures on the public. This leads to two important challenges for risk communication, first to clarify the difference between hazard and risk and, second, to help avoid such unwanted effects by designing better communication about precautionary measures.