Background: To investigate the correlates of youth tobacco use in terms of nonsmoking adolescents' openness to future smoking, a secondary analysis of the 2000 and 2004 Indiana Youth Tobacco Survey (IYTS) was conducted. Methods: A representative sample of 1416 public high school students in grades 9-12 and 1516 public middle school students in grades 6-8 (71.44% and 72.53% response rates, respectively) were surveyed in 2000, and 3433 public high school students and 1990 public middle school students (63.04% and 65.44% response rates, respectively) were surveyed in 2004. Results: Seventy-four percent of students in 2000 were not open to future smoking and 77% were not open in 2004. The adolescent cohort in 2004 became more exposed to antitobacco messages and less exposed to protobacco messages and environmental tobacco smoke (ETS) compared with their counterpart in 2000. Whereas gender, grade, race/ethnicity, and exposure to antitobacco messages were insignificant predictors for openness to future smoking, exposure to ETS either in homes or in cars was a strong predictor for openness to future smoking (the higher the exposure to ETS, the more open to future smoking) in both unadjusted and adjusted multivariate models. Exposure to protobacco messages had a greater effect on openness to future smoking than exposure to antitobacco messages. The rate of transition from openness to future smoking to tobacco use initiation is higher among white adolescents than among minority adolescents. Conclusions: More efforts should be made to reduce adolescents' exposure to ETS and protobacco messages. (Contains 3 tables.)