Prior research has indicated that pubertal development and peer associations are important determinants of adolescent smoking behavior. However, more remains to be learned about "why" these variables matter or how they may be related to one another in ways that lead to the initiation of smoking. Using contractual data from the National Longitudinal Study of Adolescent Health, we consider the relationship between early pubertal development and associations with close friends who smoke, and smoking initiation among male youths. The results of the study reveal a link between advanced pubertal development and the initiation of smoking among boys, but we discover that the effect is indirect, mediated by a greater propensity of sexually mature males to forge friendships with peers who smoke. We also find that this propensity is greatest among the youngest adolescents in the sample, suggesting that the age of the respondent conditions the effects of pubertal development on the formation of friendships with peers who smoke.