This study investigates whether peers are a contributing factor in childhood body-weight outcomes. Using an instrumental variables method on exogenously assigned peers, we find that the weight of peers within the same grade and school significantly impacts body mass index (BMI) z-score of an individual student. A typical student’s BMI z-score increases when facing heavier peers. The size of the peer-effect, however, is very modest. For a percentage point increase in the proportion of obese students in the same grade, a typical student’s BMI z-score increases by only 0.00341 standard deviations.