Over the 5-month study, over 94 percent of eligible youth participated in the interviews each month, and all interviewed youth provided urine specimens for analysis, suggesting the feasibility of voluntary urinalysis drug testing within the juvenile justice system. High participation rates were probably facilitated by the confidential, nonadversarial approach taken and youths' belief in the value of such testing. Youths showed high rates of self-reported and tested illicit drug use, particularly marijuana/hash hish. Concomitant with these high levels of drug use, youths showed high rates of educational, psychological, and family problems. While the correspondence between self-reported cannabinoid use and cannabinoid-positive test results was high, 59 percent of youth who denied cocaine use prior to detention tested positive for this drug. Cannabinoid-positive youth were more likely to be white, older, and referred for nondrug felony offenses than were cannabinoid-negative youth. Cocaine-positive youth had higher drug felony and status offense referrals and more self-reported physical abuse than cocaine-negative youth. Both drug-positive groups had higher drug and property delinquency and status offense referrals than did nonusers. Implications for practice are discussed. 12 tables, 7 notes, and 72 references.