Objective: To test cognitive-developmental, social-cognitive, motivational, and cognitive hypotheses about the psychological mechanisms underlying children's risk perception. Method: Youth (N = 1315) ranging from 9 to 17 years of age completed measures assessing adolescent egocentrism, personal experience with four negative health events, how much they worried about the health events, and their perceived skill for event-related activities. The measures were completed twice, 12 months apart. Results: Lacking personal experience with and worrying less about health threats were significant predictors of more optimistically biased risk perception a year later. Conclusions: The lack of experience with and not worrying about serious health consequences may desensitize children to potential health risks. Clinical applications for health education programs are discussed. © Society of Pediatric Psychology 2005; all rights reserved.