The study obtained research interviews from 3,086 male juvenile arrestees ages 9 to 18 years held in booking facilities in 11 cities throughout the United States. The study used urine analysis as a gold standard to evaluate the validity of self-report drug use among the juvenile arrestees. Willingness to report substance use varied according to the type of substance, the time frame for substance abuse reports, and the characteristics of the juveniles asked to provide the reports. Youth were particularly reluctant to disclose recent use of cocaine and heroin. Race/ethnicity and willingness to disclose other substance use were the most important predictors of cocaine use disclosure among those testing positive for this drug. Race/ethnicity differences in validity were evaluated in the context of other recent epidemiological findings from surveys of drug use in the United States. The article discusses implications for the measurement of drug use in criminal justice samples. Tables, notes, references