Allogeneic bone marrow transplantation (BMT) has been used in the treatment of poor risk acute lymphoblastic leukaemia (ALL) for over 20 years. Over this period results have improved and indications for treatment have become more clearly defined. Over 60% of adults and over 70% of children with poor risk ALL in first remission, and 30-40% of patients in second remissions can be expected to achieve long term leukaemia-free survival. Factors implicated in the cure of ALL by BMT are the myeloablative preparative regimen, a graft-versus-leukaemia effect, and post transplant chemotherapy. Improved results of chemotherapy have changed the perceived indications for BMT in ALL and have led to controversy over the best treatment approach. However there is good evidence to show that BMT offers a better chance of leukaemia free survival in certain very poor risk categories. These include Philadelphia chromosome positive ALL, remission induction failures, and children in second remission who relapse after adequate chemotherapy. Particular issues in the use of BMT in ALL are the prevention and management of extramedullary leukaemia, the treatment of relapse following BMT, and the prevention and monitoring of late effects. In the future the use of unrelated donors, and a continuing fall in transplant related morbidity and mortality will extend the use of BMT in poor risk ALL.