This study was performed to assess the probability of finding HLA-matched donors for Korean and Japanese patients from unrelated marrow donor registries of both countries. A simulation study of donor search was carried out using the donor pools of the Korean Marrow Donor Program (KMDP) with 10,244 and the Japan Marrow Donor Program (JMDP) with 53,411 HLA-A, -B, -DR typed donors. The records of a total of 184 actual Korean patients and 1,302 simulated Japanese patients were searched and A, B, DR-matched donors were found for 28% of Korean and 76% of Japanese patients from the KMDP and JMDP pools, respectively. Of those patients who could not find matched donors in the registry of their own country, 30% of Koreans could find matched donors in the JMDP and 10% of Japanese in the KMDP pools. It can be concluded that future international collaboration between Korea and Japan would be very effective for unrelated bone marrow transplantations.