The mediators of favorable multisystemic therapy (MST) outcomes achieved at 12 months postrecruitment were examined within the context of a randomized effectiveness trial with 127 juvenile sexual offenders and their caregivers. Outcome measures assessed youth delinquency, substance use, externalizing symptoms, and deviant sexual interest/risk behaviors; hypothesized mediators included measures of parenting and peer relations. Data were collected at pretreatment, 6 months postrecruitment, and 12 months postrecruitment. Consistent with the MST theory of change and the small extant literature in this area of research, analyses showed that favorable MST effects on youth antisocial behavior and deviant sexual interest/risk behaviors were mediated by increased caregiver follow-through on discipline practices as well as decreased caregiver disapproval of and concern about the youth's bad friends during the follow-up. These findings have important implications for the community-based treatment of juvenile sexual offenders. (Contains 3 tables.)