Some children with relapsed or high-risk acute leukaemia have an improved outcome if they have an allogeneic stem cell transplant, preferably from a sibling or well-matched unrelated donor. However, some children do not have these options or there is an urgent need to proceed to transplant because of disease status. We have investigated the role of haploidentical family members as donors in 34 patients with acute leukaemia (median age 11 years, range 1-16 years). Patients were conditioned with cyclophosphamide and total body irradiation (14.4 Gy in eight fractions) and received T-cell depleted peripheral blood stem cell grafts with a median CD34 cell dose of 13.8 x 10(6)/kg (range 4.2-35.1) and 0.7 x 10(4) CD3-positive cells/kg. The actuarial survival at 2 years was 26% (13-41%, 95% CI). Eight patients have survived disease-free with a median follow up of 62 months. They have good performance status and a median lymphocyte count of 1.8 x 10(9)/l. Relapse (14 patients) and adenoviral (six patients) or fungal infections (four patients) were the major causes of death. Haploidentical stem cell transplantation can produce medium term disease-free survival in a proportion of children with high-risk or relapsed acute leukaemia. None of the nine patients with acute myeloid leukaemia not in remission have survived.