To investigate the leukemia relapse of AL patients after HLA haploidentical bone marrow transplantation (HLA HBMT), 2 relapsed leukemia patients received HLA HBMT were studied, peripheral blood simples and bone marrow smear were examined, morphologic change of bone marrow cells was observed, while the HLA genotype and chromosome karyotye were analyzed by PCR and routine G-banding methods, respectively. The results indicated that the two cases were diagnosed primarily as acute lymphocytic leukemia (common cell subtype) and acute megakaryocytic leukemia, in which chromosome abnormalities or activation of protooncogene in leukemic cells were observed. The complete hematopuietie reconstitution of donor origin was obtained in these 2 cases after HLA HBMT, but the leukemic cells in these 2 leukemia patients were confirmed to be donor origin after relapse, their blood groups and HLA genotype were found to be originated from donor. These 2 relapsed leukemia patients were diagnosed as acute lymphocytic leukemia (B cell subtype) and acute megakaryocytic leukemia. It is suggested that high-dose of immunosuppressive agents used in transplantation may contribute to leukemia relapse of donor origin in these patients. Abnormalities in hematopoietic microenvironment may be also involved in the leukemia development. Donor-cell leukemia after allogeneic hematopoietic stem cell transplantation can be an ideal model to investigate the related events in human leukemogenesis.