This paper extends the recent literature on the influence of peers on adolescent weight on three new fronts. First, based on a survey of secondary school students in Spain in which peers are formed by nominated classmate friends, we find a more powerful positive and significant causal effect of friends' mean BMI on adolescent BMI than previous US-based research. These results are in line with international data which show that peer group contact tends to vary across countries. Our findings cover a large set of controls, fixed effects, the testing of correlated unobservables, contextual influences and instrumental variables. Second, social interactions are identified through the property of intransitivity in network relationships. Finally, we report evidence of a strong, positive effect of peer pressure on several subgroups of adolescents in an attempt to study their vulnerability to social influences.