This article (a) describes a mental model underlying initial evaluations of illness signs and (b) reports an experiment demonstrating the model's utility by showing how the model represents evidence of defensiveness among people who test positively for a sign of illness. The model consists of a set of cognitive elements that people consider to evaluate the threat represented by a sign of possible illness. Seventy-two undergraduates were led to believe that they tested positively or negatively on a saliva test for a fictitious risk factor for a disease. In addition, half the participants were told about the existence of a simple preventive treatment for the disease, whereas the others were not. Subsequently the participants answered questions about elements of the threat-appraisal model. Analysis of their responses reveals evidence of defensiveness on several elements of the model. Those testing positively for the risk factor, especially those uninformed about its treatment, minimized threat by (a) increasing their estimates of the false-positive rate of the test, (b) decreasing their estimates of the seriousness of the risk factor, and (c) decreasing their estimates of the extent to which the disease itself is life-threatening. Applications of the model to actual illness threats and the relation between threat-related judgments and health-related behavior are discussed.