The effectiveness and transportability of multisystemic therapy (MST) were examined in a study that included 118 juvenile offenders meeting DSM-III-R criteria for substance abuse or dependence and their families. Participants were randomly assigned to receive MST versus usual community services. Outcome measures assessed drug use, criminal activity, and days in out-of-home placement at posttreatment (T2) and at a 6-month posttreatment follow-up (T3); also treatment adherence was examined from multiple perspectives (i.e., caregiver, youth, and therapist). MST reduced alcohol, marijuana, and other drug use at T2 and total days in out-of-home placement by 50% at T3. Reductions in criminal activity, however, were not as large as have been obtained previously for MST. Examination of treatment adherence measures suggests that the modest results of MST were due, at least in part, to difficulty in transporting this complex treatment model from the direct control of its developers. Increased emphasis on quality assurance mechanisms to enhance treatment fidelity may help overcome barriers to transportability.