AbstractThis study represents a report on unexpected positive collateral benefits in the reduction of aggression. The treatment, for compliance across three classes of requests, resulted in the spontaneous reduction in non‐targeted aggressive behavior across settings and time. An institutionalized adult male who was severely retarded and chronically aggressive served as the subject. Archival investigator‐ and facility staff‐generated reports of aggressive episodes, based upon an operational definition in use for over a year at the time of the study, were used as dependent measures. A withdrawal design was used to demonstrate the collateral treatment effects upon aggression. The subject received positive treatment for his noncompliance within a multiple baseline across behavior classes design. Fifteen requests were presented to the subject in random order during each treatment session, compliant behavior was reinforced using praise and physical contact. Results of the compliance training showed immediate and marked increases in compliance with a collateral decrease in the incidents of aggression. While aggression increased during the nonintervention follow‐up phase of the study, it remained significantly lower than baseline