Aggression Control Therapy (ACT), a treatment programme developed in the Netherlands for violent forensic psychiatric patients with a conduct disorder or antisocial personality disorder, was investigated in two studies. In the first study, the personality traits and problem behaviours of these patients and a normative Dutch population were compared, and then the traits and behaviours of patients who completed the ACT were compared with those who dropped out. In the second study, the ACT was evaluated by comparing pre-treatment, post-treatment, and follow-up data. Two control conditions were added: a waiting-list period for outpatients and a control group for inpatients. The patients who received ACT were psychologically unstable, egoistic, and prone to anger. They reported little social anxiety when exhibiting limit-setting behaviour (e.g. giving criticism) but tended to avoid approaching behaviour (e.g. giving a compliment). Results suggested that ACT diminished aggressive behaviour but did not change socially competent behaviour. The limitations of the two studies are mentioned and suggestions for further research into the effects of ACT are presented.