This article argues that no version of the precautionary principle can be reasonably applied to decisions that may lead to fatal outcomes. In support of this strong claim, a number of desiderata are proposed, which reasonable rules for rational decision making ought to satisfy. Thereafter, two impossibility theorems are proved, showing that no version of the precautionary principle can satisfy the proposed desiderata. These theorems are directly applicable to recent discussions of the precautionary principle in medicine, biotechnology, environmental management, and related fields. The impossibility theorems do not imply, however, that the precautionary principle is of no relevance at all in policy discussions. Even if it is not a reasonable rule for rational decision making, it is possible to interpret the precautionary principle in other ways, e.g., as an argumentative tool or as an epistemic principle favoring a reversed burden of proof.