Early onset of behavioural problems has lasting negative effects on a broad range of lifetime outcomes, placing large costs on individuals, families and society. A number of researchers and policy makers have argued that early interventions aimed at supporting the family is the most effective way of tackling child behaviour problems. This study forms the economic component of a randomised evaluation of the Incredible Years programme, a programme aimed at improving the skills and parenting strategies of parents of children with conduct problems. Our results show that the programme provides a cost-effective way of reducing behavioural problems. Furthermore, our cost analysis, when combined with a consideration of the potential long-run benefits, suggests that investment in such programmes may generate favourable long-run economic returns.