The prevalence of last year use of alcohol, cigarettes, marijuana and cocaine in the U.S. population and conditional prevalence of a proxy measure of last year dependence among last year users of each drug class were assessed as a function of age, gender and ethnicity. Analyses were based on three aggregated waves (1991, 1992 and 1993) of the nationally representative samples of the general population aged > or = 12 interviewed in the National Household Surveys on Drug Abuse (n = 87915). An approximation of DSM-IV drug-specific last year dependence for each drug class was derived from self-reported symptoms of dependence, data on frequency and quantity of use and drug-related problems reported for the last year. Descriptive and multivariate analyses were conducted. The inclusion of cigarettes among the drugs, the large number of cases and the wide age range of respondents (> or = 12) enable us to make drug, age, gender and ethnic comparisons not otherwise possible in any other data set. The proxy measure of dependence, however, has limitations. The five major findings are that: (1) nicotine is the most addictive of the four drugs we examined; (2) among female last year users of alcohol and marijuana, adolescents are significantly more at risk for dependence than any other age group of women; (3) conditional prevalences of last year dependence on alcohol, marijuana and cocaine are higher among adolescent females than adolescent males but significantly different only for cocaine; (4) among adults, the rates of dependence are higher among males than among females for alcohol and marijuana, but lower for nicotine; and (5) among last year users, whites are more likely than any other ethnic group to be dependent on nicotine and blacks to be dependent on cocaine.