Using the National Longitudinal Study of Adolescent Health and multi-level modeling, we examine the role of social comparison with schoolmates in adolescent girls’ weight control. Specifically, we focus on how girls’ own weight control is influenced by the body sizes and weight-control behaviors of their schoolmates. Our findings suggest that comparisons with similar others (in this case, girls of a similar body size) appear to have the strongest association with individual girls’ reports of trying to lose weight. For example, the odds that an overweight girl is engaged in weight control increase substantially when many overweight girls in her school are also trying to lose weight. This study highlights how schools play an important role in shaping girls’ decisions to practice weight control and demonstrates how social comparison theory improves our understanding of how health behaviors are linked to social contexts.