Smoking onset has 4 levels, with a "susceptibility" level preceding early experimentation. This study assessed the predictive validity of smoking susceptibility in a longitudinal study of a nationally representative sample of 4,500 adolescents who at baseline reported never having puffed on a cigarette. At follow-up 4 years later, 40% of the sample had experimented with smoking, and 8% had established a smoking habit. Baseline susceptibility to smoking, defined as the absence of a firm decision not to smoke, was a stronger independent predictor of experimentation than the presence of smokers among either family or the best friend network. However, susceptibility to smoking was not as important as exposure to smokers in distinguishing adolescents who progressed to established smoking from those who remained experimenters at follow-up.