Regarding substance use, analyses showed that the evidence-based treatments of adolescent substance abuse (i.e., multisystemic therapy [MST] and contingency management [CM]) were associated with significantly greater reductions in sibling substance use than were the standard community treatments. On the other hand, and in contrast with hypotheses, siblings of youths in the juvenile drug court conditions did not show greater reductions in delinquency than did the siblings of youths under the intervention of family court with community services. Under the MST and CM conditions, parents were taught how to administer random substance-use screens, how to provide consequences and conduct functional analyses based on the results of the screens, and how to assist the adolescent in self-management planning. Perhaps these caregiver skills were transferred to the siblings of the adolescent client either directly or indirectly. It is also possible that the favorable sibling outcomes were produced by treatment-related changes in family functioning and the adolescent client's peers. The sample of 70 siblings averaged 14.4 years old; 50 percent were boys; 71 percent were African-American; and 27 percent were White. Measures of sibling substance use and delinquency were collected at pretreatment, 4 months, 12 months, and 18 months. Multilevel longitudinal models were used to determine whether changes in sibling substance use and delinquency paralleled the treatment effects observed for their substance-abusing delinquent brothers and sisters in the juvenile court study. 2 tables, 1 figure, and 35 references