Most information on the prevalence of drug use comes from self-report surveys. The sensitivity of such information is cause for concern about the accuracy of self-report measures. In this study, self-reported drug use in the last 48 hr is compared to results from biological assays of saliva samples from 371 young adults entering clubs. The relationship between self-reports and drug presence in oral fluid was determined for three substances as follows: cocaine, marijuana, and amphetamine. Forty-one percent of the participants with drugs detected in their oral fluids reported no use in the last 48 hr. The significance of these results is discussed. (Contains 9 tables.)