The participants were randomly assigned to four different self-report methods of assessing drug use: (1) individual interview in the workplace, (2) group-administered questionnaire in the workplace, (3) telephone interview, and (4) individual interview off the worksite. Urine specimens were collected and analyzed on all 928 participants, and hair analysis was conducted on 307 participants. Results revealed that although self-reports produced the highest prevalence rate of drug use, analyses combining the results of the three assessment methods revealed that the actual prevalence rate was approximately 50 percent higher than the estimate produced by self-reports. In addition, the group-administered questionnaire produced prevalence rates that were approximately half those of the other self-report methods. Findings cast doubt on the validity of self-reports as a means of estimating drug use prevalence and suggest the need for multiple assessment methods. Figures, tables, author biographies and photographs, and 21 references (Author abstract modified)