A total of 116 consecutive patients, including 82 cases of hematologic malignancies and 34 cases of severe aplastic anemia (SAA), were treated by allogeneic bone marrow transplantation (BMT). The conditioning regimens and post-graft immunosuppressive agents were as described by Thomas in Seattle and Santos in Baltimore. The incidence of grades II-IV acute GVHD in patients with hematologic malignancies with a full HLA match, with one locus and with two loci mismatches was 6.3% (4 of 63), 0% (0 of 11) and 37.5% (3 of 8), respectively. None of SAA patients developed grade II-IV acute GVHD. The low incidence of acute GVHD after BMT among Chinese patients may be associated with the use of isolation in laminar airflow rooms and a relatively low level of genetic diversity in histocompatibility antigens. This is an interesting issue for further study.