Attempted human allogeneic marrow transplants in the 1950's and 60's were largely unsuccessful. In the past two decades the probability of success has improved steadily depending on the type and stage of disease. All marrow transplant teams have observed that the results for children are better than for adults. Long-term survival and apparent cure rates range from about 90 percent for non-malignant diseases transplanted early to 15 percent for patients with advanced leukemia. Most marrow transplants have involved an HL-A matched sibling donor but, more recently, through the worldwide marrow donor registries a matched unrelated volunteer marrow donor can be found for many patients without a family donor. Current research involves new preparative regimens for elimination of malignant cells, better prevention of graft-versus-host disease (GVHD), and the use of hematopoietic growth factors and cytokines. Autologous transplants, which use the patient's own marrow, are increasing. The hematopoietic stem cell, which is responsible for marrow regeneration after a transplant, has been isolated and characterized. Stem cells for transplantation can now be obtained from the peripheral blood after mobilization of these cells by chemotherapy or hematopoietic growth factors.