This case-control study investigated factors associated with late initiation (i.e., initiation after the age of 17 years) of smoking among young women. The most significantly elevated odds ratios for late initiation were having a significant other who smoked and having friends who found smoking acceptable at initiation. Peers, especially significant others, played an important role in smoking initiation among young women. Prevention efforts should focus on strategies that reduce the acceptability of smoking in the social environment.