Contrasts between self-reported use and hair assay results are examined with special attention to individual differences, interviewers' ratings of truthfulness for each respondent, and whether or not the respondent qualified as being substance dependent. Results show that the likelihood of admitting cocaine use was positively related to having received drug misuse treatment before, and negatively related to being Hispanic. Evidence is also presented which indicates that the lower levels of disclosure among Hispanics may have been the result of poor communication. Interviewers proved to be relatively adept at discerning which respondents were truthful. Some results suggest that, despite considerable underreporting among the overall sample, subjects who actually qualified as being substance dependent were much more likely to honestly report cocaine use. Tables, references, abstracts in Spanish and French