The authors applied person-environment transaction theory to test the acquired preparedness model of eating disorder risk. The model holds that (a) middle-school girls high in the trait of ineffectiveness are differentially prepared to acquire high-risk expectancies for reinforcement from dieting or thinness; (b) those expectancies predict subsequent binge eating and purging; and (c) the influence of the disposition of ineffectiveness on binge eating and purging is mediated by dieting or thinness expectancies. In a three-wave longitudinal study of 394 middle-school girls, the authors found support for the model. Seventh-grade girls' scores on ineffectiveness predicted their subsequent endorsement of high-risk dieting or thinness expectancies, which in turn predicted subsequent increases in binge eating and purging. Statistical tests of mediation supported the hypothesis that the prospective relation between ineffectiveness and binge eating was mediated by dieting or thinness expectancies, as was the prospective relation between ineffectiveness and purging. This application of a basic science theory to eating disorder risk appears fruitful, and the findings suggest the importance of early interventions that address both disposition and learning.