Antisocial behaviour (conduct disorder) is the commonest psychiatric problem in childhood. An interview was developed to measure the direct and indirect costs arising from antisocial behaviour over a range of domains, including behaviours inside and outside the home. Data were collected for a pilot sample of 10 children aged 4–10 years who had been referred to child and adolescent mental health services. The average cost across the 10 families was £15,382 a year, ranging from £5411 to £40,896 per family. The greatest cost fell on the families themselves (average nearly £5000 per year) and the education authority (average nearly £5000 per year); and there were significant costs for the health service, social services and the Benefits Agency.