Seventeen bone marrow transplants were undertaken on 15 patients with leukemia or aplastic anemia using marrow from closely matched (phenotypic or five out of six HLA-A, B and DR antigen matched related and unrelated) donors. Donors were siblings (four), parents (seven), aunt (one), great aunt (one) or matched unrelated (two). When compared with transplants using matched sibling donors, survival was not different (51.4 +/- 13.4% vs. 48.1 +/- 9.6%; p = 0.87) but transplant-related complications and morbidity were higher as follows: graft-versus-host disease (GVHD) (87% vs. 15%; p less than 0.001), interstitial pneumonitis (59% vs. 14%; p less than 0.003), days in hospital (51 vs. 26; p less than 0.001), and chronic transplant related morbidity 50% vs. 11%; p + 0.033). The age of donors who were closely matched was significantly greater than that of their recipients (29.7 +/- 13.9 years vs. 8.1 +/- 3.1 years; p less than 0.001) and was associated with poorer transplant outcome. Median transplant-related complication-free survival for patients receiving transplants from age non-disparate donors was 53 months (range 18-86 months) compared with 12 months (range 2-42 months) for age disparate donors (p = 0.028). Transplants from closely matched donors were undertaken in the ratio of one to every three matched donors, indicating the importance of this source of marrow in a transplant program.