This is a cross-sectional study conducted to examine the associations between the composition of friendship networks and health-related behaviors of 988 adolescents (11-18 years old) from Colombia. Participants were asked to self-report behaviors and to nominate their best friends. "Egocentric network analysis" (i.e., analysis of personal networks formed by egos and alters) was conducted to identify "network composition" (i.e., the proportion of network members with a particular attribute or behavior), and associations were analyzed with logistic regression models. Results show that network composition was associated with individual physical activity by boys and with consumption of fruits and vegetables, smoking, and drug use by girls. Regardless of gender, network composition was associated with screen time, fast-food consumption, excessive alcohol consumption, and having had sexual intercourse. These findings highlight the role of the social and behavioral environment in health-related behaviors in adolescents.