This present study investigated the genetic and environmental influences on the associations between adolescents' peer characteristics (i.e., peer college orientation, and peer delinquency) and smoking in young adulthood. We used longitudinal data from the Nonshared Environment and Adolescent Development (NEAD) project. Parents' reports on adolescents' peer characteristics and adolescents' self-reports on smoking in young adulthood were examined. Genetic and environmental influences on each construct as well as on the association between the two were analyzed. Findings showed that genetic and nonshared environmental influences contributed to peer college orientation and smoking status. Genetic, shared and nonshared environmental influences contributed to peer delinquency. Further, genetic and nonshared environmental influences contributed to the association between adolescents' peer college orientation and smoking in young adulthood. Genetic and shared environmental influences contributed to the association between adolescents' peer delinquency and smoking in young adulthood. In conclusion, the present study showed that genetic influences contributed to adolescents' peer characteristics and, in addition, genetic factors mediated the association between peer characteristics and smoking.