Models of individual preventive action generally view behavior change as the result of a decision about the costs and benefits of different options. Costs are often viewed as a combination of the likelihood and severity of potential harm, although individuals' perceptions of these factors often do not reflect their objective characteristics. In fact, several studies of health and safety hazards and other negative life events indicate that most people are unrealistically optimistic regarding the likelihood of harm. Studies have also found associations between perceptions of vulnerability and protective actions, although the relationships between these two variables are unclear. Thus, the effectiveness of interventions suggested to reduce unrealistic optimism regarding susceptibility to AIDS or other risks is unknown. Tables, notes, and 76 references.