It seems obvious that 2 key attributes of health hazards, their perceived probability and perceived severity, do not act independently on the motivation to engage in protective behavior. If a health problem is perceived to have no chance of occurring, there should be no interest in acting against it, regardless of how serious it might be. Nevertheless, researchers seldom observe the expected interaction between probability and severity. A case study approach was used to examine how probability and severity combine to influence interest in protection. Ratings of motivation to act, probability, and severity for 201 hazards were collected from 12 participants, and data were analyzed for each person separately. Analyses revealed the expected Probability x Severity interaction. Additional calculations showed why it is difficult to detect this interaction using between-subjects designs. The data also revealed that people are surprisingly insensitive to variations in hazard probability when probabilities are in the moderate to high range.