This chapter describes the current role of unrelated donor stem cell transplantation (UD-SCT) in the management of leukaemia. The available data are scant and incomplete and there are few randomized studies comparing UD-SCT with alternative therapies. Patients with many of the leukaemias require prolonged follow-up after allogeneic SCT to determine whether they are cured; the registry-based comparisons that have been initiated reflect the results achievable some years ago and may not help us in deciding what is best in 2001. In addition, new therapies such as ST1571, even though the long-term outcome of patients treated with this agent is uncertain, may affect which patients with chronic myeloid leukaemia we decide to recommend for transplant. The focus here is on acute and chronic myeloid leukaemia, acute lymphoblastic leukaemia and chronic lymphocytic leukaemia, as well as the myelodysplastic syndromes. Patient selection, conditioning strategies, comparison with other therapies, timing of transplant and the major causes of treatment failure are discussed, and there is an exploration of where improvement will come from.