The precautionary principle is a useful strategy for decision-making when physicians and patients lack evidence relating to the potential outcomes associated with various choices. According to a version of the principle defended here, one should take reasonable measures to avoid threats that are serious and plausible. The reasonableness of a response to a threat depends on several factors, including benefit vs. harm, realism, proportionality, and consistency. Since a concept of reasonableness plays an essential role in applying the precautionary principle, this principle gives physicians and patients a decision-making strategy that encourages the careful weighing and balancing of different values that one finds in humanistic approaches to clinical reasoning. Properly understood, the principle presents a worthwhile alternative to approaches to clinical reasoning that apply expected utility theory to decision problems.