In a treatment-effect framework using Add Health data, we investigate whether adolescents gain weight when increasingly exposed to obesity in their social networks. We find that weight gain can be a reaction to an increase, but not a decrease, in exposure to obesity that is based on social ties, not geographic proximity. Taking an endogenous growth perspective on the prevalence of obesity, we thus attempt to reveal a mechanism through which obesity may potentially develop into a sweeping epidemic. Our results also suggest an uphill battle against the obesity epidemic, and we recommend that its prevention be a high priority.