Although acute myeloid and lymphoid leukemia initially respond to conventional chemotherapy, most patients relapse and succumb from their disease. For the past 30 years, efforts at intensifying induction or postremission conventional chemotherapy have met with limited success. Of all the therapies examined, allogeneic hematopoietic stem cell transplantation has achieved the best outcomes, with the first reports of cure 25 years ago. Most phase III studies have failed to demonstrate a clear advantage of allografting over chemotherapy because of significant risk of transplant-related mortality. However, there is reason for optimism based on the identification of high-risk groups and from improvements in the prevention and management of transplant-related complications. These accomplishments, along with a recent increase in the donor pool through the availability of unrelated donors and cryopreserved umbilical cord blood stem cells, continue to make allografting the most promising curative modality for acute leukemias.