The effects of multisystemic therapy (MST) in treating violent and chronic juvenile offenders and their families in the absence of ongoing treatment fidelity checks were examined. Across 2 public sector mental health sites, 155 youths and their families were randomly assigned to MST versus usual juvenile justice services. Although MST improved adolescent symptomology at posttreatment and decreased incarceration by 47% at a 1.7-year follow-up, findings for decreased criminal activity were not as favorable as observed on other recent trials of MST. Analyses of parent, adolescent, and therapist reports of MST treatment adherence, however, indicated that outcomes were substantially better in cases where treatment adherence ratings were high. These results highlight the importance of maintaining treatment fidelity when disseminating complex family-based services to community settings.