Abstract Purpose To describe the incidence and identify predictors of smoking initiation in young adults. Methods Data were collected in self-report questionnaires in 22 cycles over 13 years in a prospective cohort investigation of 1,293 students recruited in 1999–2000 from all grade 7 classes in a convenience sample of 10 high schools in Montreal, Canada. Participants were 12.7 years of age on average at cohort inception and 24.0 years of age in cycle 22. Independent predictors of smoking initiation in young adulthood (post–high school) were identified in multivariable logistic regression analysis using generalized estimating equations. Results Of 1,293 participants, 75% initiated smoking by cycle 22. Of these, 44%, 43%, and 14% initiated before high school, during high school, and in the 6 years after high school, respectively. The incidence density rate of initiation was .33, .13, .14, .11, and .12 initiation events per person-year in grade 7, 8, 9, 10, and 11, respectively, and .05 post–high school. Independent predictors of smoking initiation in young adults included alcohol use, higher impulsivity, and poor academic performance. Conclusions A total of 14% of smokers who initiated smoking before age 24 years did so after high school. The predictors of initiation in young adults may provide direction for relevant preventive interventions.