As most patients are not fortunate enough to have an HLA-matched sibling to use as a bone marrow donor, attention has focused on the use of either HLA-matched but unrelated donors or HLA-mismatched family members. With the maturation of the field of histocompatibility testing, it is now possible to quantitate with relative precision the degree of disparity between patient and donor. In general, it appears that with respect to histocompatibility differences between donors other than HLA-matched siblings, there is an increased incidence of acute graft-versus-host disease, with the risk correlated with the degree of histoincompatibility. However, the overall disease-free survival is not always adversely affected, as a graft-versus-leukemia effect may counterbalance the increased death rate from graft-versus-host disease. To find donors for most patients, efforts are under way to recruit a large number of unrelated volunteers into the National Marrow Donor Program.