Many patients with hematological malignancies potentially curable by bone marrow transplantation are not considered for transplantation because an HLA identical family or unrelated donor is unavailable. For these patients the only curative option is a transplant from a partially matched family donor. Such transplants are feasible but are less successful than matched sibling donor transplants. The main problems with mismatched transplants are graft rejection, graft-vs-host disease, and regimen-related mortality. This restricts the use of mismatched transplants to patients less than 45 years at high risk of dying from the hematological malignancy.