This article describes the findings of a 10-week pilot programme of aggression replacement training (ART) in an Australian youth justice, custodial setting. Five male subjects (17–18 years old) completed pre- and post-treatment self-report measures of aggression, social skills, and cognitive distortions typically associated with violent and antisocial behaviour. As expected, results showed a significant reduction in overall aggression and improved social skills from before to after assessment, but the predicted reduction in cognitive distortions was not supported. Treatment outcomes were examined relative to the specific multi-modal (cognitive, behavioural, and affective) components of ART. Hence, results are discussed in relation to a cognitive behavioural theory of change, which underpins ART. A single case study is presented to illustrate qualitative change throughout participation in ART. Overall the results provide preliminary support for the efficacy of ART in an Australian context, and further investigation with a larger, multi-informant sample is warranted.