One hundred forty-seven consecutive patients with leukemia, myelodysplastic syndrome, or aplastic anemia were treated by marrow grafts from genotypically HLA-identical siblings (n = 122) or HLA- haploidentical family members (n = 25). Haploidentical recipients differed from their donors for no more than one HLA locus on the nonshared haplotype. All were given postgrafting immunosuppression with a combination of methotrexate and cyclosporine. In a randomized study we explored whether prednisone administered from day 0 through 35 along with methotrexate/cyclosporine could improve prevention of acute graft- versus-host disease (GVHD). The GVHD incidence in patients not given prednisone was comparable with that previously reported with methotrexate/cyclosporine. Unexpectedly, significant increases in acute and also chronic GVHD were seen in HLA-identical recipients administered prednisone, but not in the small number of patients administered HLA-nonidentical grafts. However, the resultant increase in transplant-related mortality in patients administered prednisone was offset by an increase in leukemic relapse in patients not administered prednisone, presumably related to the absence of a graft-versus- leukemia effect. Therefore, overall disease-free survival of the two groups of patients was comparable, with slightly more than 50% of the patients being alive at more than 2 years after transplantation. We speculated that prednisone adversely affected GVHD prophylaxis, interfering with methotrexate's cell cycle-dependent suppression of donor lymphocyte proliferation in response to host antigens. In a pilot study we explored whether beginning prednisone on day 15, after completion of methotrexate administration, would avoid this adverse effect. The GVHD incidence in patients administered methotrexate/cyclosporine along with “late” prednisone was comparable with that in patients not administered prednisone. We conclude that methotrexate/cyclosporine is effective in decreasing the incidence of grade II through IV GVHD, and that the addition of prednisone to this regimen is not beneficial in recipients of HLA-identical marrow grafts.