A large body of literature has explored the validity of self-reported drug use among criminal populations. These works have typically revealed strong underreporting of recent illicit drug use. To date, however, no studies have compared the validity of self-reported drug-using behaviors between adult and juvenile offenders. The authors overcome this limitation by assessing the validity of self-reported recent drug use with data collected from 1,425 adult and 551 juvenile Los Angeles arrestees surveyed through the Arrestee Drug Abuse Monitoring (ADAM) Program in 1997. Kappa statistics were computed and tests for specificity and sensitivity were conducted utilizing urinalysis as the reference standard. Results suggest that adult arrestees are more inclined to underreport their recent use of illicit drugs. Implications for drug use research are assessed in light of the current findings.