Anger is related to violence prior to hospitalization, during hospitalization, and after discharge. Meta-analyses have established treatment efficacy in reducing anger, but few studies have addressed whether reduced anger leads to lowered aggressive behavior. This study concerns individually-delivered anger treatment, specialized for offenders with intellectual disabilities, delivered twice weekly for 18 sessions to 50 forensic hospital patients. Assessments involved patient self-report of anger, staff ratings of anger and aggression, and case records of assaultive incidents. Physical assault data were obtained from records 12 months pre-treatment and 12 months post-treatment. Significant reductions in assaults following treatment were found by GEE analyses, controlling for age, gender, length of stay, IQ, and pre-hospital violence. Following treatment, physical attacks reduced by more than half, dropping from approximately 3.5 attacks per patient 6 months prior to treatment, versus approximately 1 attack per patient in the 6–12 month interval post-treatment. In hierarchical regressions, controlling for IQ, reduction in physical assaults was associated with pre-to post-treatment change in anger level. These findings buttress the efficacy of anger treatment for patients having histories of violence and have significance for patient mental health, hospital staff well-being, therapeutic milieu, hospital management, and service delivery costs.