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OBJECTIVES: To describe the development of a prognostic tool to identify adolescents at risk for transitioning from never to ever smoking in the next year. METHODS: Data were drawn from the Nicotine Dependence in Teens study, a longitudinal investigation of adolescents (1999 to present). A total of 1294 students initially age 12 to 13 years were recruited from seventh-grade classes in 10 high schools in Montreal. Self-report questionnaire data were collected every 3 months during the 10-month school year over 5 years (1999-2005) until participants completed high school (n = 20 cycles). Prognostic variables for inclusion in the multivariable analyses were selected from 58 candidate predictors describing sociodemographic characteristics, smoking habits of family and friends, lifestyle factors, personality traits, and mental health. Cigarette smoking initiation was defined as taking even 1 puff on a cigarette for the first time, as measured in a 3-month recall of cigarette use completed in each cycle. RESULTS: The cumulative incidence of cigarette smoking initiation was 16.3%. Data were partitioned into a training set for model-building and a testing set to evaluate the performance of the model. The final model included 12 variables (age, 4 worry or stress- related items, 1 depression-related item, 2 self-esteem items, and 4 alcohol- or tobacco- related variables). The model yielded a c-statistic of 0.77 and had good calibration. CONCLUSIONS: This short prognostic tool, which can be incorporated into busy clinical practice, was used to accurately identify adolescents at risk for cigarette smoking initiation. [ABSTRACT FROM AUTHOR]