Bone marrow transplantation is an experimental approach to the treatment of patients with acute leukemia, aplastic anemia, and other neoplastic and genetic diseases. To date, long-term disease-free survival has been achieved in a small proportion of carefully selected patients with resistant acute leukemia. While results are not optimal, they are acceptable in late stage patients where there are no effective alterates. Major problems in marrow transplantation for leukemia include tumor resistance and a spectrum of immunologic complications including GVHD, immunodeficiency, and interstitial pneumonitis. Potential approaches to these problems have been suggested. Progress in any one area would have a substantial impact on improving survival and extending the applicability of marrow transplantation to patients at an earlier stage of their disease.