Results of 470 bone marrow transplants from related donors other than genotypically HLA-identical siblings (alternative related donors) were analysed to identify factors associated with transplant outcome and to determine whether T cell depletion improved results. As compared to 3648 transplant from HLA-identical siblings, alternative related donor transplants were associated with increased graft failure, increased acute graft-versus-host disease (GVHD), and lower disease-free survival. The likelihood of adverse outcome correlated with increasing donor-recipient HLA-disparity. In multivariate analysis of alternative related donor transplants, donor age greater than or equal to 30 years, (relative risk [RR] 1.7, p less than 0.006), intermediate and advanced leukemia (RR 1.5 and 1.6, p less than 0.01 and p less than 0.003), infection pretransplant (RR 1.7, p less than 0.005) and 2- and 3-locus donor-recipient HLA-disparity (RR 1.3, p less than 0.04) were associated with increased risks of treatment failure. The 2-year probability of leukemia-free survival after alternative related donor transplants (n = 43) with none of these adverse prognostic features was 44% (95% confidence interval 28-59%) compared to 56% (95% confidence interval 52-59%) for similar patients receiving HLA-identical sibling transplants (n = 868, univariate p less than 0.03). T cell depletion increased graft failure and decreased acute GVHD after alternative related donor transplants but did not improve leukemia-free survival.