Working knowledge or vocabulary of drug slang, as a manifestation of learned behavior, might help predict or explain risk of starting to use cannabis in early adolescence. To study this issue, an epidemiologic sample of 1,255 11- to 12-year-olds was assessed for knowledge of cannabis slang terms in 1992, with follow-up in 1993 and 1994. The basic design is that of a prospective and longitudinal study, with recruitment of an epidemiologic sample of children as they entered primary school in a single metropolitan area, also with baseline assessments and randomization to intervention conditions, and subsequent multiple waves of follow-up assessment. Youths assessed in 1992 and who were familiar with terms such as blunts, Mary Jane, roach, and herb were more likely to start using cannabis within the subsequent two years of the follow-up interval, as compared to other youths (estimated relative risk, RR=11.0; 95% CI 3.6-33.7; P<.001). A youth's working vocabulary may signal important variations in health- and disease-related behavior, such as illegal drug use.