The aim of this study was to examine the effect of treatment characteristics on recidivism in a forensic youth-psychiatric outpatient clinic. The treatment offered comprised functional family therapy (FFT), individual cognitive behavioural therapy (CBT), or CBT in combination with parent training. Some of the youth additionally participated in aggression replacement training (ART). FFT and ART were implemented as a trial version, meaning that most therapists had not received formal training yet. Treatment characteristics related to recidivism were length of treatment, type of treatment, number of sessions, and the therapist. The longer the period of treatment and the greater the number of sessions, the higher the recidivism, even after controlling for risk of recidivism based on static risk factors. Juveniles who participated in ART reoffended more often than juveniles who had not participated in such training. Given the fact that FFT and ART were not well-implemented trial versions, it can be concluded that poorly implemented treatment leads to poor outcomes.