Human marrow transplantation has resulted in observations of fundamental significance in understanding both aplastic anemia and acute leukemia. For example, the observation that transplanted marrow can grow successfully in patients with aplastic anemia indicates that the disease is due to a defect in the marrow precursor cells and not in the marrow microenvironment. Similarly, the observation of recurrent leukemia in donor cells has important implications. Nonetheless, marrow transplantation is sufficiently established therapeutically to be considered the treatment of choice for patients with severe aplastic anemia, and a realistic alternative for patients with recurrent acute leukemia. We suggest that patients be managed with regard to marrow transplantation according to the general approach outlined in Table 3. Marrow transplantation and histocompatibility typing are available at increasing numbers of institutions throughout the world. More and more patients with either severe aplastic anemia or recurrent acute leukemia should have marrow transplantation available to them when it is indicated as part of optimal management of these no longer hopeless diseases.