Recent evidence indicates that higher smoking rates among young adults in the United States may be related in part to increased initiation during young adulthood. The tobacco industry, restricted from overtly targeting adolescents, appears to be focusing on young adults. Thus it is important to estimate the percentage and identify the characteristics of the young adult population (aged 18-29 years) at risk for future smoking. This group would include all current established smokers (smoked at least 100 cigarettes in their lifetime), current experimenters, former established smokers, and former experimenters at risk for smoking again, as well as never-smokers who do not rule out future smoking. Using data from the population-based 2002 California Tobacco Survey (N = 9,364 young adults aged 18-29 years), we designated groups of young adults at risk and identified factors associated with risk. Altogether, 86.7+/-1.5% of current and former established smokers were at risk for future smoking, and 59.6+/-4.3% of former established smokers were at risk. Over half (52.2+/-2.0%) of all experimenters but only 9.0+/-1.2% of never-smokers were at risk. Overall, 43.0+/-1.2% of the young adult population was at risk. Although different demographic and other factors were associated with risk among former established smokers, experimenters, and never-smokers, three factors were consistently associated in each group: Younger age, not having a smoke-free home, and going to bars and clubs. We conclude that the group of young adults at risk for future smoking is sizable and presents a fertile target for tobacco industry efforts to promote smoking. Counter-efforts might include promotion of smoke-free environments.