Relapsed disease is the most common cause of death in children with hematological malignancies. Patients who fail high-intensity conventional chemotherapeutic regimens or relapse after stem cell transplantation have a poor prognosis. Toxicity from multiple therapies and elevated leukemic/tumor burden usually make these patients ineligible for the aggressive chemotherapy regimens required for conventional stem cell transplantation. Alternative options are needed. One type of treatment being explored is called haploidentical transplant.