The influence of parents and other important adults on adolescent substance use is becoming recognized as a salient topic of research. A study designed to assess the impact of adult substance use on adolescents' progression through increasingly more advanced stages of substance use is reported here. Latent Transition Analysis was used to estimate the probabilities, conditional on exposure to adult substance use, of adolescents' (N=5,242) belonging to one of nine progressively more advanced stages of adolescent substance use. The probabilities of adolescents moving from one stage in the onset process to another, again, conditional on adult substance use, were also estimated. Results suggest that substance use by adults is a potent risk factor for adolescent substance use experimentation. Adolescents who reported exposure to adult use of alcohol, tobacco, or marijuana are more likely to be further advanced in the onset process at each of the junior high school years, grades 7 through 9. Findings regarding exposure to adult use of marijuana were most pronounced. It was not known whether the increased risk was due to modeling of substance use behavior, ineffective parenting due to substance use, the availability of substances, or a genetic predisposition toward substance use. (RJM)