Data from a longitudinal study of detained juveniles are examined to address this issue. Results indicate that self-reported delinquent behavior and alcohol use prior to initial interview are key predictors of reported delinquent behavior during the 10- to 15-month follow-up period. However, self-reported drug use and urinalysis test results, measured at initial interview, and prior delinquent behavior are significant predictors of the use of marijuana/hashish and cocaine over time. The research and policy implications of these findings are drawn. 13 notes, 9 tables, and 71 references (Author abstract)