Background:Little evidence exists regarding the effectiveness or cost-effectiveness of alternative treatment services in the field of child and adolescent psychiatry. Aims:To assess the cost-effectiveness of a home-based social work intervention for young people who have deliberately poisoned themselves. Method: Children aged ⩽16 years, referred to child mental health teams with a diagnosis of deliberate self-poisoning were randomly allocated to either routine care (n=77) or routine care plus the social work intervention (n=85). Clinical and resource-use data were assessed over six months from the date of trial entry. Results: No significant differences were found between the two groups in terms of the main outcome measures or costs. In a sub-group of children without major depression, suicidal ideation was significantly lower in the intervention group at the six-month follow-up (P=0.01), with no significant differences in cost. Conclusions: A family-based social work intervention for children and adolescents who have deliberately poisoned themselves is as cost-effective as routine care alone.