Transplantation of syngeneic (donor is a monozygous twin) or allogeneic (donor is an HLA-identical sibling) marrow provides the opportunity for aggressive antileukemic therapy without regard to marrow toxicity. Until 1975, marrow transplantation was carried out only after failure of all other therapy. Consequently, most patients were in advanced relapse. Six of 16 recipients of syngeneic marrow and 13 of 100 recipients of allogeneic marrow are still in remission after 5. 5-10 years 3, 7]. An actuarial survival curve of the first 100 patients grafted in Seattle after conditioning with...