The analysis reveals that in adolescence and young adulthood, blacks report lower lifetime rates of drug use than whites or Hispanics. At ages 35 and over, blacks report consistently higher rates than whites. This latter pattern appears to be gradually spreading into younger age groups. These data raise at least two paradoxes. One results from the contrast with the ethnic distributions of drug-related cases recorded in medical, legal, and treatment institutions, where blacks are overrepresented. Another is that the lower rates of drug use reported by black youths compared with white youths contrast with black youths' higher participation in problem behaviors other than drug use. Interpretations of within-population differences are methodological and include underreporting of drug use, exclusion of at-risk groups from samples, and spuriousness of the associations between ethnicity and drug use. Interpretations of the overrepresentation of blacks in institutional samples include lack of sample comparability, bias in who comes into contact with institutions, and differential severity of drug involvement among users from different ethnic groups. Tables, figures, note, and 54 references