The focal point of this paper is the transition from drug use to drug dependence. We present new evidence on risk for starting to use marijuana, cocaine, and alcohol, as well as risks for progression from first drug use to the onset of drug dependence, separately for each of these drugs. Data from the National Comorbidity Survey (NCS) were analyzed. The NCS had a representative sample of the United States population ages 15-54 years (n ϭ 8,098In an earlier report of evidence from the National Comorbidity Survey, our research group described some interesting features about the comparative epidemiology of drug dependence. For example, as a summary population average value for the estimated millions of Americans age 15-54 years who had tried cocaine at least one time by the early 1990s, about one in six had become dependent upon cocaine (about 16-17% NO . 4 marijuana at least one time, about one in 11 had become dependent upon it (about 9%). By comparison, among persons who had tried alcoholic beverages at least once, about one in 6 or 7, or 15%, had become alcohol dependent (Anthony et al. 1994). In the present study, we extend this look at the comparative epidemiology of drug dependence, but here our focus is upon estimation of the age-specific and time-specific risks of progression from first drug use to dependence, separately for marijuana, cocaine, and (for comparison) alcoholic beverages (hereinafter, 'alcohol').Prior studies have conveyed estimates of age-specific risk for first alcohol use and alcohol dependence, as well as risk estimates for initiation of illicit use and dependence on controlled drugs in general (e.g., Kandel and Logan 1984;Eaton et al. 1989;Warner et al. 1995;Chen and Kandel 1995;Johnson and Gerstein 1998;Perkonigg et al. 1999;DeWit et al. 2000). The role of early onset of drug use and progression to initial and problematic use of other drugs also has been studied in some detail (e.g., Kandel 1985;Anthony and Petronis 1995;Grant 1998;Grant and Dawson 1998). However, what is especially novel about the present study is its new look at both the cumulative and instantaneous risk of drug dependence in relation to time elapsed since first use of marijuana and cocaine, with alcohol for comparison.Whereas epidemiological studies of this type generally are regarded as valid sources of evidence, it is possible that some critics might call into question the validity or reliability of drug dependence assessments obtained in large-sample survey research as compared with what can be obtained via intensive clinical study of smaller samples (e.g., see Anthony et al. 1985;Brugha et al. 1999). Nonetheless, at least with respect to the drug dependence syndromes, recent empirical research provides evidence that epidemiologic studies can and do provide generally valid and reliable estimates of the occurrence of these conditions, as well as their corresponding ages of onset (e.g., see Prusoff et al. 1988;Langenbucher et al. 1994;Shillington et al. 1995;Wittchen et al. 1989Wittchen et al. , 1998Wittchen et ...