The study supports the explanation that marijuana use, in itself, increases one's probability of other illicit drug use. The finding suggests that marijuana users are three to five times more likely to use other illicit drugs compared with peers who do not use marijuana. The alternative explanation that the same set of variables correlate with both marijuana use and subsequent use of other illicit drugs was not supported. After adjusting statistically for the influence of variables associated with strain, social-bonding, and differential-association theories, marijuana users were still significantly more likely than nonusing peers to use other illicit drugs. The authors advise, however, that these findings are not evidence that adolescent marijuana users are inevitably going to use other illicit drugs. The results only show that using marijuana may increase the probability that a given individual will use other illicit substances. This study confirms the findings of many other studies which show that most marijuana users do not use other illicit substances. Suggestions for further research are offered. Data for this study came from the first, third, and fifth waves of the NYS. The NYS involved a nationally representative panel of 1,725 American adolescents (ages 11-17). Researchers examined not only the degree to which respondents reported using a broad array of illicit substances, but also the degree to which they experienced high amounts of strain (strain theory), exhibited strong relationships with conventional others (social-bond theory), and associated with peers who engaged in high levels of substance use (differential-association theory). 5 tables, 1 figure, 44 references, and appended strain, social bonding, and differential-association scale items