BMT has been successfully utilized in clinical medicine for nearly 25 years. Successful accomplishment of BMT has severely taxed the research and clinical capabilities of medical scientists, but the attainment of a cure for fatal disease has amply justified the efforts expended. The major problems still to be solved include better conditioning regimens, better control of GVHD, and enlargement of donor pools. The control of problems created by this therapy demands a thorough understanding of the immune system. As this knowledge increases, useful manipulation of immunity to control the hundreds of other diseases caused by aberrant immune processes should become possible. Soon, genetically engineered marrow transplants should be possible.