To prevent GVHD in BMT from unrelated donors, the matching of HLA between patient and donor is crucial. The appearance of acute GVHD was studied in 51 patients with hematological malignancies who were transplanted with non-T cell purged marrow from HLA-A,B and DR compatible unrelated donors with the assistance of the Tokai Marrow Donor Bank of Nagoya, Japan. Probability of grade II-IV acute GVHD was 32.0% and of grade III-IV acute GVHD was 17.0%. HLA-class II antigen compatibility showed a good correlation with the occurrence of acute GVHD. When the percentage relative response (RR) of MLC between patient and donor (GVHD vector) was < or = 5, grade II-IV acute GVHD was found in only 7.7% of patients (n = 16) and no severe grade III-IV GVHD occurred. Among patients with 6-10% RR (n = 10), 25.9% showed grade II-IV GVHD and 14.3% grade III-IV GVHD. Among patients with > or = 11% RR (n = 20), however, the incidence of grade II-IV acute GVHD reached 51.8% and that of grade III-IV acute GVHD 36.2%. These reactivities of MLC reflected the compatibility of HLA-DRB1 and DPB1. The fact that the incidence of acute GVHD in BMTs from HLA-A,B,DR compatible Japanese pairs was found to be lower than in the USA may be due to less diversity of the genetic background in Japan.