This study evaluated the effectiveness of a group intervention with the mothers of aggressive, non-compliant children (aged three to six years). The intervention consisted of a parenting program, Helping Encourage Affect Regulation. Mothers who attended the program were compared with a waitlist control group. The sample consisted of 35 children and their mothers. Post-treatment assessment showed significant differences in parent-reported externalizing behavior in children, parenting knowledge, sense of competence and attitudes towards parenting their child in the Helping Encourage Affect Regulation program compared with the waitlist control group. Findings are discussed with a focus on the process of change that the group intervention is likely to have facilitated in the parents. (Contains 4 tables and 1 figure.)