Objectives - Anger is a significant predictor and activator of violent behaviour in patients living in institutional settings. There is some evidence for the value of cognitive-behavioural treatments for anger problems with people with intellectual disabilities. In this study, a newly designed treatment targeted at anger disposition, reactivity, and control was provided to intellectually disabled offenders with aggression histories living in secure settings. Design - About forty detained patients with mild-borderline intellectual disabilities and histories of serious aggression were allocated to specially modified cognitive-behavioural anger treatment (AT group) or to routine care waiting-list control (RC group) conditions. Methods - AT group participants received 18 sessions of individual treatment. The AT and RC groups were assessed simultaneously at 4 time points: screen, pre- and post-treatment, and at 4-month follow-up using a range of self- and staff-rated anger measures. The effectiveness of the treatment was evaluated using ANCOVA linear trend analyses of group differences on the main outcome measures. Results - The AT group's self-reported anger scores on a number of measures were significantly lower following treatment, compared with the RC wait-list condition, and these improvements were maintained at follow-up. Limited evidence for the effectiveness of treatment was provided by staffs' ratings of patient behaviour post-treatment. Conclusions - Detained men with mild-moderate intellectual disabilities and histories of severe aggression can successfully engage in, and benefit from, an intensive individual cognitive-behavioural anger treatment that also appears to have beneficial systemic effects.