Responsive–Aggression Regulation Therapy (Re-ART) Outpatient is a cognitive behavioral–based intervention for adolescents and young adults (16-24 years) with severe aggressive behavioral problems. This pilot study (N = 26) examined the level of program integrity (PI; that is, the delivery of the intervention as it is originally intended) of Re-ART. We also investigated the pre- and post-test changes in several outcome variables, and the relation between the level of PI and these changes. Participants were recruited from three different outpatient forensic settings. Results showed that the PI of half of the treatments was not sufficient (e.g., the intensity of the program was too low and some standard modules were not offered). In addition, this pilot study demonstrated that sufficient PI was related to positive changes in aggression, cognitive distortions, social support, coping (reported by therapist), and distrust (responsiveness to treatment).