Background: Aggression is a significant problem amongst people with intellectual disabilities (ID), particularly those residing in hospital settings. Anger is related to aggression in secure services working people with ID, and the effectiveness of psychological interventions in reducing anger has been demonstrated in this population. However, no studies have systematically examined whether levels of aggression reduce following anger treatment with people with ID detained in secure settings. Method: This programme evaluation study concerns individually delivered cognitive anger treatment delivered to 50 patients (44 men and 6 women) with mild to borderline ID, delivered twice weekly for 18 sessions in a specialist forensic hospital service. Aggressive incidents and physical assault data were obtained from records 12 months pre-treatment and 12 months post-treatment. Results: Following completion of treatment, the total number of aggressive incidents recorded in patients' files fell by 34.5%, and the post-treatment reduction in the number of physical assaults was 55.9%. Analysis of the data partitioned into 6-month blocks over the 24-month study period showed that significant reductions in aggressive and violent incidents occurred in the assessment intervals following anger treatment. Conclusions: These findings reinforce the efficacy of cognitive behavioural anger treatment for detained patients with ID and histories of aggression; and despite its methodical limitations the study indicates the ecological validity of this treatment approach.